Sara Paterson-Brown, in Basic Science in Obstetrics and Gynaecology (Fourth Edition), 2010

The lymphatic system

Lymphatic vessels

The extracellular tissues of the body are constantly gaining fluid and debris (from capillary leakage, cell death, etc.) and the function of the lymphatics is to remove this and return it to the venous circulation. The lymphatic capillaries have the same basic structure as vascular capillaries but their distribution is not uniform throughout the body. The lymphatics in the limbs tend to be superficial, while those of the viscera tend to drain via channels on the posterior abdominal and thoracic walls.

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The lymphatic vessels return the lymph to the venous system via two main channels:

The right lymphatic duct drains the right thorax, upper limb, head and neck

The thoracic duct drains all lymph from the lower half of the body.


The pre- and para-aortic lymphatics drain into the cisterna chyli which is an elongated sac-like vessel that lies over the body of L1 and L2 behind the inferior vena cava and between the aorta and the azygous vein. It becomes the thoracic duct as it ascends through the diaphragm at the level of T12. It starts on the right side of the oesophagus, but as it ascends through the thorax the thoracic duct passes behind the oesophagus (at T5) to reach its left side, then superiorly it passes over the left subclavian artery and the dome of the left pleura to drain into the confluence of the left subclavian with the left internal jugular veins.

Lymphatics, like blood vessels (and unlike somatic nerves), can cross the midline, but in contrast they pass to and from lymph nodes (afferent and efferent lymphatics) and they comprise an anastomosing low-pressure system.

Lymphatic tissue

These comprise concentrations of lymphocytes and occur in mucosal and submucosal collections in the gut (e.g. Peyer's patches in the ileum) as well as in the thymus, the spleen and lymph nodes themselves.

The anatomical clinical importance of this system relates to the drainage patterns of each group of nodes, which is summarized in Table 5.1, but also described for the individual organs in their relevant regional anatomy sections.


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A. Kaipainen, D.R. Bielenberg, in Encyclopedia of the Eye, 2010

Structure and Function of the Lymphatic System

The lymphatic system forms a one-way route, carrying lymph from the periphery of tissues through the thoracic duct or the right lymphatic duct into the venous blood. These two main lymphatic ducts are connected with the venous system at the junction of the left internal jugular vein and the left subclavian vein and at the veins of the right jugulo-subclavian confluence, respectively. However, other potential lymphaticovenous communications (e.g., iliac and renal areas) may become functional when lymphatic pressure rises or in a pathological situation.

Beyond its main function, the drainage of extravasated tissue fluid back to the venous circulation, the lymphatic system serves several other functions which include transport for immune defense and absorption of lipids from the intestine. Nearly all vascularized tissues, except the central nervous system (CNS), are invested with lymphatic vessels. The CNS lacks lymphatics since it has a blood-neural barrier that is less permeable than other tissues and is immune-privileged. In the eye, lymph drainage is present in the conjunctiva, sclera, and choriocapillaris, while there is no lymph drainage for the anterior chamber, vitreous cavity, subretinal space, or cornea.


In the peripheral tissues, lymphatic vessels and blood vessels are found in close proximity, yet the two systems never intermix (Figure 1). Lymphatic functions are reflected in the particular structure of the lymphatic vessels. A main characteristic is the discontinuity of the basement membrane at the interface between the lymphatic endothelium and the surrounding connective tissue that facilitates active fluid transport. In some tissues, including intestine, lung, and skin, lymphatic vessels completely lack a basement membrane. A second major characteristic is the tight connection of the lymphatic endothelial cells (LECs) to the surrounding matrix through anchoring filaments (AFs). It has been proposed that tissue expansion due to excess interstitial fluid tightens the AFs, which pull on the lymphatic capillaries, thereby creating gaps between the LEC to increase the intake of fluid. A third characteristic of the lymphatic vessels is that valves in the vessel wall are already present at the level of capillaries, unlike in the venous blood system where they are found only in venules and larger vessels. These valves ensure unidirectional flow of the lymphatic fluid, which starts in the blind-ended capillaries. Furthermore, LECs are significantly larger than the blood endothelial cells; this enables elongation of the cells to accommodate the tissue stretch (see Table 1 for a list of differences between lymphatic vessels and blood vessels).


*

Blood vesselsLymphatic vessels
Circular systemUnidirectional system
Artery → capillary → veinCapillary → collector → TD → vein
Formed E6.5–9.5 (mice)Sprout from vein at E9.5–12.5 (mice)
Capillaries have BMCapillaries have discontinuous or lack BM
SMC may surround capillariesCapillaries lack SMC
No anchoring filamentsCapillaries have anchoring filaments
Flow is dictated by heart beatFlow is dictated by interstitial pressure
No valves in capillariesCapillaries have valves
Blood inside the vesselsLymph inside the vessels
Contain all the hematopoietic cellsContain immune cells and no RBC
Retina has blood vesselsRetina lacks lymphatic vessels

BM, basement membrane; RBC, red blood cells; SMC, smooth muscle cells; TD, thoracic duct.


David L. Hirsch, Michael J. Spink, in Current Therapy In Oral and Maxillofacial Surgery, 2012

Cervical Lymphatics

Henri Rouvière schematically described the lymphatic drainage of the head and neck as two concentric narrowing funnels draining caudally to the thoracic duct (left) and lymphatic duct (right). This paradigm is oversimplistic but is still taught in schools today. Regional metastases from the oral cavity often drain to Robbins levels I to III, hence the rationale for supraomohyoid neck dissection. Skip metastases are possible, and the tongue and soft palate are often discovered to have bilateral regional metastases on final pathologic evaluation. Table 53-1 illustrates the anatomic levels of the neck and their importance.

Predictable drainage of the oral cavity to the first echelon of the lymphatic basin does exist. However, as a result of data from large clinical outcome studies and the ability of lymphoscintigraphy to map sentinel nodes, surgeons now recognize that drainage can be on an individual basis. Skip metastases to level IV in lateral tongue SCC and retropharyngeal drainage of the soft palate follow this paradigm. Another caveat lector is that previously operated necks may have hitherto undiagnosed, nascent, or recurrent metastases. Therefore, lymphatic drainage can be unpredictable after surgery, and clinically positive (cN+) or negative (cN0) nodes with micrometastases may go undetected.

Radiologists combine nodal critical size and morphology to determine “suggestive” cervical adenopathy. Suspicion for regional metastases is high in the setting of OSCC if nodes display a central hypointensity consistent with central necrosis; if they are round and not kidney bean shaped, which represents expansion; if the surrounding fascial plane is obliterated, which signifies tissue necrosis or fixation; if their dimensions are greater than 15 mm at level II and greater than 10 mm elsewhere; or if a spiculated periphery indicative of extracapsular spread is present. This last characteristic is a significant poor prognostic indicator for OSCC.


Alfonso López, Shannon A. Martinson, in Pathologic Basis of Veterinary Disease (Sixth Edition), 2017

Chylothorax.

The accumulation of chyle (lymph rich in triglycerides) in the thoracic cavity (Fig. 9-116) is a result of the rupture of major lymph vessels, usually the thoracic duct or the right lymphatic duct. The clinical and pathologic effects of chylothorax are similar to those of the other pleural effusions. Causes include thoracic neoplasia (the most common cause in human beings but a distant second to idiopathic cases in dogs), trauma, congenital lymph vessel anomalies, lymphangitis, dirofilariasis, and iatrogenic rupture of the thoracic duct during surgery. The source of the leakage of chyle is rarely found at necropsy. When the leakage of chyle occurs in the abdominal cavity, the condition is referred to as chyloabdomen. Cytologic and biochemical examination of fluid collected by thoracocentesis typically reveals large numbers of lymphocytes, lipid droplets, few neutrophils in chronic cases, and high triglyceride content.


Robert G. Carroll PhD, in Elsevier's Integrated Physiology, 2007

Lymphatics

Lymphatics are a network of endothelial tubes that merge to form two large systems that enter the veins. Lymph from the right side of the head, right trunk, and right arm drains into the right lymphatic duct. Lymph from the remainder of the body drains into the thoracic duct, which empties into the thoracic vena cava.

Terminal lymphatics (Fig. 8-3) lack tight junctions, allowing large proteins (and metastasizing cancer cells) to enter the circulatory system through the lymphatic system. Lymph composition closely resembles interstitial fluid composition. In the GI tract, lymphatics allow digested fats to enter the circulation. Lymph is propelled by (1) massaging from adjacent muscle, (2) tissue pressure, and (3) contraction of the lymph vessels. Valves ensure that the flow of lymph is toward the vena cava. Over 24 hours, the volume of lymph flow in the body is equal to approximately 5 L, the same as the total blood volume. Lymph is filtered in lymph nodes before progressing back to the circulation.


Mary Anne Jackson, J. Christopher Day, in Principles and Practice of Pediatric Infectious Diseases (Fifth Edition), 2018

Lymphatic Drainage of the Lungs and Pleura

As shown in Fig. 18.1, lymph from the thoracic viscera (heart, pericardium, lungs, pleura, thymus, and esophagus) traverses one of three possible sets of nodes before entering the thoracic duct or right lymphatic duct. Anterior mediastinal nodes are located anterior to the aortic arch, innominate veins, and large arterial trunks leading from the aorta. They receive afferents from the thymus and pericardium, the sternal nodes, and the thyroid gland.

Posterior mediastinal nodes lie dorsal to the pericardium and adjacent to the esophagus and descending aorta. They receive afferents from the esophagus, dorsal pericardium, diaphragm, and convex surface of the liver. Middle or mediastinal nodes drain the lungs and pleura. Lymphatic drainage of the lungs is composed of superficial and deep plexuses. The superficial plexus lies beneath the visceral pleura. Lymph flows around the border of the lung to enter the bronchopulmonary (hilar) nodes. The deep plexus accompanies branches of the pulmonary vessels and ramifications of the bronchi throughout the lungs.

Lymphatic drainage of the lung passes through four sets of lymph nodes (Table 18.1). Intrapulmonary lymph nodes are located within the lung, chiefly at the bifurcations of the larger bronchi. Bronchopulmonary or hilar nodes are located at the pulmonary hilus at the site of entry of the main bronchi and vessels. Tracheobronchial nodes are divided into superior and inferior groups. The superior group lies in the obtuse angle between the trachea and bronchi on both sides. The inferior, or subcarinal, group lies under the carina at the tracheal bifurcation. The fourth group, the tracheal or paratracheal nodes, lies beside and somewhat anterior to the trachea. A fifth group of lymph nodes of importance in the drainage of the lungs is the inferior deep cervical (scalene or supraclavicular) chain, which is located over the lower portion of the internal jugular vein, just above the clavicle and usually under the scalenus anterior muscle. The apical pleurae drain directly to these deep cervical nodes, as do the paratracheal chains. A finding of supraclavicular lymphadenopathy should lead to investigation for intrathoracic or intra-abdominal pathology.

Ultimately, all lymph from the lungs and pleurae reaches the tracheobronchial and paratracheal lymph nodes. Generally, lymph from the lungs flows from left to right, a probable explanation for the preeminence of right upper paratracheal and supraclavicular lymphadenopathy in infectious pulmonary processes, particularly tuberculosis. Lymph from the left lower lobe (and usually also the lingula) flows from the hilar nodes to the lower tracheobronchial nodes, and then to the right paratracheal nodes. Lymph from the right hilar nodes travels to the right paratracheal nodes (see Table 18.1).

Lymph vessels from the paratracheal nodes join with lymph trunks from the anterior mediastinum to form the right and left bronchomediastinal trunks. These trunks then join with the lymphatic trunks from the supraclavicular nodes to form the right lymphatic duct and left thoracic duct.


Bruce H. Culver, Robb W. Glenny, in Clinical Respiratory Medicine (Fourth Edition), 2012

Lymphatic Circulation

Pulmonary lymphatics are not found in alveolar walls but originate in interstitial spaces at the level of the respiratory bronchioles and at the pleural surface, then follow the bronchovascular bundles to the hila. The lymph flows through the right lymphatic duct and the thoracic duct into the right and left brachiocephalic veins. The total flow from the lungs is quite low under normal conditions (less than 0.5 mL/minute in experimental animals) but can increase many-fold with pulmonary edema. The lymphatics have valves to prevent backflow and can generate sufficient pressures to maintain flow when systemic venous pressure is as high as 20 cm H2O.


Mary Anne Jackson, J. Christopher Day, in Principles and Practice of Pediatric Infectious Diseases (Fifth Edition), 2018

Anatomy and Function of Lymphoid Tissue

The lymphoid system is composed of an extensive capillary network that drains lymph into elaborate systems of collecting vessels. The collecting vessels merge and empty lymph into the bloodstream by way of the thoracic duct at its entry into the left subclavian vein or by the right lymphatic duct, which empties into the right subclavian vein. Specialized lymphatic structures interspersed along the collecting vessels include the tonsillar tissues of the Waldeyer ring, the thymus, the spleen, mucosa-associated lymphoid nodules, and lymph nodes (Table 16.1).

The Waldeyer ring of lymphoid tissue that surrounds the oropharyngeal isthmus and the opening of the nasopharynx into the oropharynx is uniquely positioned to interact with foreign material entering the nose or mouth. The ring is formed superiorly by the midline pharyngeal (adenoid) tonsil, which is located in the roof of the nasopharynx, and inferiorly by the lingual tonsils in the posterior third of the tongue. On either side of the pharynx, the lateral pharyngeal bands of lymphoid tissue connect the adenoid to the tubal tonsils of Gerlach at the openings of the eustachian tubes and to the faucial (palatine) tonsils. Smaller aggregates of lymphoid tissue in this area include the posterior pharyngeal granulations and the lymphoid tissue within the laryngeal ventricle.

Small submucosal lymphoid nodules located throughout the respiratory, gastrointestinal, and genitourinary tracts are composed of phagocytic and lymphoid cell collections without a connective tissue capsule. These nodules are ideally situated to respond to mucosal antigens.

The thymus, which is located over the superior vena cava in the anterior mediastinum, is relatively protected from antigens. Surrounded by a thin connective tissue capsule, the thymus is uniquely composed of epithelial and lymphatic elements.

The spleen is the largest lymphatic organ in the body and the only lymphatic tissue specialized to filter blood. Similar to the lymph nodes, the spleen is a component of the peripheral lymphoid system and is composed of red pulp (i.e., red blood cells) and an interior of white pulp, which contains lymphoid nodules with germinal follicles.

Normal lymph nodes are small, oval or bean-shaped bodies that are strategically located along the course of lymphatic vessels to filter lymph on its way to the bloodstream. Lymphatic vessels enter around the periphery of the nodes. Lymph filters through the cortex to the medulla of the node and exits through the hilum. Blood vessels enter and leave through the hilum, which is connected to capillaries that course through the node. During this process, lymphocytes can leave the blood and re-enter the lymphatic circulation.

Nodes are densely packed with lymphocytes that are organized loosely into cortical nodules and medullary cords by connective tissue trabeculae and lymphatic sinuses. The juxtaposition of phagocytic cells, antigen-processing cells, and lymphocytes in an area of sluggish blood flow is ideally suited to provide the first line of defense against pathogens. As lymph slowly filters through the rich reticular network, organisms are trapped and can be ingested by phagocytic cells, stimulating the release of cytokines, which recruits lymphocytes for immunologic responses. The lymph node groups in the body can be divided into the superficial and peripheral nodes, which usually are easily palpable, and the deeper groups adjacent to major vessels and viscera (see Table 16.1).


C.J. Carati, B.J. Gannon, in Encyclopedia of Respiratory Medicine, 2006

Lymphatic networks in the lung are found in close association with blood vessels and bronchi, and in the pleura. These networks anastomose at the lung surface and interlobular septa, and drain via the hilar region into the mediastinal and tracheobronchial lymphatic system, and thence to the right lymphatic duct or thoracic duct. Interstitial fluid from the alveolar walls drains into the parenchyma of the alveolar ducts, where it enters blind-ended lymphatic capillaries consisting of simple discontinuous but overlapping endothelial cells. These in turn drain into collecting lymphatic vessels that contain smooth muscle and one-way valves to help pump the lymph centrally along the network, significantly aided by respiratory and vascular movement. This drainage maintains normal tissue hydration, but is overcome in cardiovascular and lung disease resulting in fluid accumulation at the alveolar level. The lymphatics also provide a route of removal of inflammatory and pathological material, including tumor cells, which often end up in lymph nodes. Factors that can compromise lymphatic drainage from the lungs include reduced lymphatic pumping by compromised respiratory, vascular, or body movement; inhibition of lymphatic pumping by inflammatory cytokines or cells; constriction of lymphatic vessels by external forces; obstruction of the lymphatics by tumor; or increased central venous pressure. Pleural fluid drainage is via intercellular gaps called stomata in the pleural mesothelial lining, which directly connect the pleural cavities to lymphatics that drain the dependent regions of the parietal pleura.


The following code and results are an illustration of how one might do semantic consistency checking in an ontology like the FMA. We specifically focus on the lymphatic system because it will be revisited in Section 5.5.3. Here are two specific requirements for consistency and completeness in the lymphatic system:

1.

Every lymphatic chain should have efferent-to relations only to other lymphatic chains or to lymphatic vessels such as the Thoracic duct or the Right lymphatic duct.

2.

Starting with any lymphatic drainage from an organ or organ part, all paths using the efferent-to relation should end up at either the Thoracic duct or the Right lymphatic duct.


The first requirement is a slightly more stringent requirement than the current FMA specifies in the “efferent to” relation, but it is necessary in order to get meaningful pathways. In the FMA specification of the “efferent to” relation, the following are allowed classes:

Lymphatic chain

Lymphatic vessel

Anodal lymphatic tree

Lymphatic plexus

Lymph node

The Thoracic duct and Right lymphatic duct are lymphatic trunks, which are subclasses of Lymphatic vessel, so they should be allowed, along with other trunks. Anodal lymphatic trees are small networks of lymphatic vessels that directly drain structures.


The tests will be performed in a sequence of steps. Of course for fully automated checking these steps could easily be combined. First, for the lymphatic chains and vessels, we get all the instances, using the all-subclasses function previously defined.

> (setq allchains (all-subclasses "Lymphatic chain"))

("Pulmonary lymphatic chain"

 "Subdivision of pulmonary lymphatic chain"

 "Axillary lymphatic chain" "Subdivision of axillary lymphatic tree"

 "Posterior mediastinal lymphatic chain"

 "Tracheobronchial lymphatic chain"

 "Tributary of tracheobronchial lymphatic chain"

 "Left cardiac tributary of tracheobronchial lymphatic chain"

 "Brachiocephalic lymphatic chain"

 "Right cardiac tributary of brachiocephalic lymphatic chain" …)

> (setq allvessels (all-subclasses "Lymphatic vessel"))

 ("Variant lymphatic vessel" "Lymphatic capillary"

 "Tributary of lymphatic trunk" "Tributary of lymph node"

 "Superficial lymphatic vessel" "Deep lymphatic vessel"

 "Lymphatic trunk" "Incomplete right lymphatic duct"

 "Absent thoracic duct" "Absent cisterna chyli" …)


Checking the lengths of the two lists that result, it appears there are 353 chains and 670 vessels. For each of the items in the two lists, we retrieve the values in the “efferent to” slot. This is easily done by writing a function to go through each list, get the “efferent to” contents, and pair it with its chain or vessel.

(defun get-efferents (terms)

(mapcar #’(lambda (x)

(list x (efferent-to x)))

terms))


We apply it to both lists, giving the following results.

> (setq chain-slots (get-efferents allchains))

 (("Pulmonary lymphatic chain" ("Bronchopulmonary lymphatic chain"))

("Subdivision of pulmonary lymphatic chain" NIL)

("Axillary lymphatic chain"

("Subclavian lymphatic trunk" "Subclavian lymphatic tree"))

 ("Subdivision of axillary lymphatic tree" NIL)

 ("Posterior mediastinal lymphatic chain"

("Thoracic duct" "Tracheobronchial lymphatic chain"))

 ("Tracheobronchial lymphatic chain"

("Bronchomediastinal lymphatic trunk"

"Bronchomediastinal lymphatic tree"))

 ("Tributary of tracheobronchial lymphatic chain" NIL)

 ("Left cardiac tributary of tracheobronchial lymphatic chain" NIL)

 ("Brachiocephalic lymphatic chain"

("Bronchomediastinal lymphatic trunk"

 "Bronchomediastinal lymphatic tree"))

 ("Right cardiac tributary of brachiocephalic lymphatic chain" NIL)

 …)

> (setq vessel-slots (get-efferents allvessels))

 (("Variant lymphatic vessel" NIL) ("Lymphatic capillary" NIL)

 ("Tributary of lymphatic trunk" NIL)

 ("Tributary of lymph node" NIL)

 ("Superficial lymphatic vessel" NIL) ("Deep lymphatic vessel" NIL)

 ("Lymphatic trunk" NIL) ("Incomplete right lymphatic duct" NIL)

 ("Absent thoracic duct" NIL) ("Absent cisterna chyli" NIL) …)


Now we can just iterate through these lists checking if the entries match our criteria above. In every entry, the second item must be a list of only lymphatic chains or vessels, in which case we put it on a “good” list, or it is nil, meaning no information is entered, and we put it on a “not done” list, or it is some other things, in which case we put it on a “bad” list.

(defun check-efferents (termslots allowed)

"termslots is a list of chains or vessels and their efferent to

 slot values. allowed is the complete list of allowed values"

 (let (good bad not-done)

(dolist (chain termslots)

(let ((efferents (second chain)))

(cond ((null efferents) (push chain not-done))

((every #’(lambda (x)

(find x allowed :test #’string-equal))

efferents)

(push chain good))

(t (push chain bad)))))

(list good bad not-done)))


Strictly speaking there is nothing in this function about the “efferent to” relation. It can be used to check any list of (term slot-value) pairs against a list of allowed values. Here are the results for the list of chains:

> (setq chain-checks (check-efferents chain-slots

(append allchains allvessels)))

((("Left submental lymphatic chain"

("Left submandibular lymphatic chain"

"Left jugulo-omohyoid lymphatic chain"))

("Right submental lymphatic chain"

("Right submandibular lymphatic chain"

"Right jugulo-omohyoid lymphatic chain")) …)

 (("Left parasternal lymphatic chain"

("Left bronchomediastinal lymphatic tree"))

("Right parasternal lymphatic chain"

("Right bronchomediastinal lymphatic tree")) …)

 (("Left level VI lymphatic chain" NIL)

("Right level VI lymphatic chain" NIL)

("Left level V lymphatic chain" NIL) …))


We consider a slot good if its values come from either the chain list or the vessel list. Now, how many are there of each?

> (length (first chain-checks))

140

> (length (second chain-checks))

11

> (length (third chain-checks))

202


So, 140 of the chains are correct, 11 have some problem, and 202 are still not completed. If we look at the 11 that have a problem, it is apparent that these entries are mostly ones where a lymphatic tree has been entered. It seems problematic that a chain could be efferent to a subtree of the lymphatic system, although this is anatomically correct. A chain that is part of a tree connects to the tree and the lymphatic fluid flows from the chain into that branch of the tree. However, for path tracing, it is not useful, because the flow from that chain does not go through the whole tree, but only through a subset of branches. So this needs to be fixed in order to do sound path tracing. There are so few that it is relatively easy to fix this modeling problem.

> (pprint (second chain-checks))

(("Left parasternal lymphatic chain"

("Left bronchomediastinal lymphatic tree"))

 ("Right parasternal lymphatic chain"

("Right bronchomediastinal lymphatic tree"))

 ("Left tracheobronchial lymphatic chain"

("Left bronchomediastinal lymphatic trunk"

 "Left bronchomediastinal lymphatic tree"))

 ("Right tracheobronchial lymphatic chain"

("Right bronchomediastinal lymphatic trunk"

 "Right bronchomediastinal lymphatic tree"))

 ("Lymphatic chain of lower lobe of left lung"

("Left bronchopulmonary lymph node"))

 ("Lymphatic chain of lower lobe of right lung"

("Right bronchopulmonary lymph node"))

 ("Parasternal lymphatic chain"

("Bronchomediastinal lymphatic trunk"

 "Bronchomediastinal lymphatic tree"))

 ("Infraclavicular lymphatic chain"

("Subclavian lymphatic tree" "Apical axillary lymphatic chain"

 "Subclavian lymphatic chain"))

 ("Brachiocephalic lymphatic chain"

("Bronchomediastinal lymphatic trunk"

 "Bronchomediastinal lymphatic tree"))

 ("Tracheobronchial lymphatic chain"

("Bronchomediastinal lymphatic trunk"

 "Bronchomediastinal lymphatic tree"))

 ("Axillary lymphatic chain"

("Subclavian lymphatic trunk" "Subclavian lymphatic tree")))


Now the same checks can be performed on the vessels list, and the counts for the three categories come out as follows:

> (length (first vessel-checks))

13

> (length (second vessel-checks))

2

> (length (third vessel-checks))

655


On inspection of the two that are flagged as bad, the same problem appears. A tree has been entered where we would expect a trunk, vessel, or chain.

The second check, to determine whether all paths terminate at the Thoracic duct or Right lymphatic duct, is more challenging. The fact that many “efferent to” relationships are still not done makes it highly likely that there are many incomplete paths, so this check is probably premature. Nevertheless, it should be clear how to perform it. From the list of chains and vessels, one traces the paths, and examines their end points. Looking back to the example for the Soft palate, you can see that starting there, a path ends at the Jugular lymphatic trunk. However, there are two jugular lymphatic trunks, a right and a left. One goes to the thoracic duct and one goes to the right lymphatic duct. So it is correct to have no entry at the higher level of generality. This is yet another complication in checking consistency and completeness, as well as in applying the knowledge to clinical problem solving. There are many such places where anatomical structures are described as general classes, and then have more specific right and left instances (subclasses). This is very important, since geography is important and (especially with radiation therapy) one must specify on which side the entities of interest are. The presence of the general and right/left instances makes reasoning difficult to automate. There is room for some further innovation here.

So, none of the results above should be considered as deficiencies in the FMA. Rather, our checks have identified some further complexities in the model itself. Another possibility is that the path check may reveal circularities. The path tracing code described previously would not terminate in this case, and a different kind of check is needed, where one keeps track of nodes already visited and flags any returns to such nodes.

The query interface described here, while still operational, is peculiar to the FMA, and not well matched to standards and methods used widely in Semantic Web research and implementation. The FMA can also be represented as a graph structure using RDF <271>, and a generalization of the SPARQL query language <426> has been developed to support the complex queries that are becoming important in medicine <90,375>.

See more: An Increase In Net Exports Will Shift The, Reading: Aggregate Demand

Anatomy, important as it is, is sometimes viewed as a “dead” subject, with little or nothing new to be discovered. However, despite several thousand years of dissection, study, and development of terminology, it seems that much remains to be done. The FMA project, with its goal to develop a consistent computational theory of anatomy, is one of the most ambitious ontology building projects in biomedical informatics. We have learned from it that the expressivity of the metaclass idea, and the representation of relations themselves as entities, are both key elements of complex biomedical theories. Another very important realization comes out of this work as well. Biomedical informatics is not just straightforward application of well-established computer science methods. As Mark Musen has said, “ours is the discipline that cares about the content” <291>. Biomedical informatics gives formal shape to biomedical content, and in the struggle to get it right, feeds back to computer science and information science new ideas and challenges. This is exactly parallel to the relation between theoretical physics and mathematics.