D. J. DAVIS 651 



hilus of the lungs, of the liver, in the mesentery, and in the axillary, inguinal, and 

 cervical regions, etc. The oro-pharyngeal region is especially important from the 

 point of view both of normal and pathogenic bacteria and of lymphatic distribution. 

 Here are prominent accumulations of lymphoid tissue, located around and on either 

 side of the throat and covered by mucous membrane which often, as in the tonsils, 

 dips down into pockets or crypts to a depth of a centimeter or more. Considered by 

 itself, from the point of view of physiology, the oro-pharyngeal lymphoid tissue is not 

 important, nor should we expect it to be. We may look upon it as a small part of the 

 extensive system of lymphatics of the pharyngo-intestinal system, all of which taken 

 together may possess an important function, but a part and even a large part may be 

 removed without serious disturbance so far as we now know. 



Lymphatic nodes may be divided into two groups: first, the lymphatic nodes 

 found along the respiratory and intestinal canal located under the mucosa and covered 

 by a layer of loose, modified epithelium, apparently designed for absorption. Bacteria 

 are found normally deep in these nodes (Digby). They have no afferent lymph vessels 

 but an abundant supply of efferent vessels which invariably lead to a second deeper 

 set of nodes. This subepithelial lymphatic tissue is frequently arranged in prominent 

 projecting masses as in the human tonsil and adenoids and in the Peyer's patches, and 

 often the surfaces are corrugated, grooved, or pitted — the result being an increase of 

 absorbing or secreting surfaces. This may be brought about at times by an evagina- 

 tion or tubular depression, as occurs in the tonsil of certain animals like the cow. 



Lymphatic glands are recently acquired structures phylogenetically, being ap- 

 parently limited to birds and mammals. Tonsils are even more recently acquired 

 many mammals like the rat, beaver, porcupine, bat, and some others not possessing 

 them. There is good reason to believe that bacteria and infectious disease in animals 

 preceded the phylogenetic development of lymphatic structures, so that the view is 

 suggested that this striking distribution of lymphatics in the body, and especially in 

 the respiratory tract, may have been determined by the bacterial distribution; the 

 distribution in turn, as stated above, was primarily determined by the anatomical 

 and physiological conditions. 



The oro-pharyngeal lymphatic tissues are prominent masses at the portal of two 

 great systems, the respiratory and the gastro-intestinal. We do not know to a cer- 

 tainty that this location is an advantage to the body, but from the standpoint of 

 disease transmission, viruses and bacteria of various kinds evidently find it to their ad- 

 vantage to locate here; for as air, secretions, food, etc., pass back and forth over these 

 structures, aided often by such processes as coughing, sneezing, talking, and breathing, 

 they are readily carried either to the outside of the body where they may be able to 

 enter another body or deeper into the body, thereby extending their field of activities, 

 to the decided advantage of the micro-organisms in their struggle for existence. 



When one views the respiratory and alimentary tract from the lips to the rectum, 

 one observes two localities where striking accumulations of lymphoid tissue appear, 

 namely, in the region of the throat, and in the lower small intestine, about the ileo- 

 cecal valve and appendix. The intervening localities, like the stomach, duodenum, 

 etc., have lymphoid tissue, but it is irregularly distributed and far less in quantity. 

 A priori, this would indicate excessive absorption of dangerous matter in these local- 



