528 PHYSIOLOGY CHAP. 



only necessary to tetauise the lingual nerves for a long time, and 

 with brief interruptions, in order to produce conspicuous tumefac- 

 tion of the half of the tongue corresponding to the side stimulated, 

 associated with dilatation of the arterial and venous vessels. 

 Marcacci has shown that the effect depends principally upon the 

 pronounced formation and accumulation of lymph (oedema), rather 

 than on hyperaemia. After protracted tetanisation of the lingual 

 nerve, he saw not only that the lymphatics of the tongue dilated, 

 but also that a large lymphatic gland which is in direct relation 

 with them, and lies near the submaxillary gland, swelled and 

 increased in weight. Since, as has been seen, the rise of arterial 

 pressure is not of itself enough to determine any great increase of 

 filtration through the capillaries, we hold that the effect depends 

 on the extension of nerve influence in this case to the lymph- 

 forming elements of the lingual tissue, which, when excited, pour 

 a more copious flow of lymph into the lymphatic spaces. The same 

 thing is seen on exciting the chorda tympani, which innervates the 

 submaxillary gland, but with the difference that in this case the 

 lymph which is more abundantly formed does not, after transform- 

 ation into the saliva of the glandular cells, flow back into the 

 lymphatic system, but is canalised iu the excretory ducts of the 



gland. 



All tissues that are in relation with the lymphatic system are 

 more or less lymphagenic in a wide sense, i.e. they pour into the 

 lymphatic system and thence into the blood system a part at least 

 of their elaboration or waste products, thereby contributing to the 

 formation of the lymph or modification of its composition. Among 

 these, more particularly, are the so-called lymphoid tissues in 

 general, the follicles and glands attached to the lymphatics, the 

 red bone-marrow, the thymus, and the spleen. 



VII. Lymphoid (or Adenoid) Tissue is the name given to such 

 tissues as consist essentially of branching cells and fibres of 

 connective tissue which are so interconnected as to constitute a 

 network with very fine meshes, within which the leucocytes are 

 enclosed in great numbers. Diffuse in form, with no circumscribed 

 boundaries, the lymphoid tissue is found in the mucosa of the 

 respiratory passages, throughout the intestinal tract, in the marrow 

 of bones, etc. In the sharply-defined form of rounded nodules the 

 size of a small pin's head, lymphoid tissue appears in the so-called 

 solitary follicles, which are found in large numbers in the intestinal 

 mucosa, especially in its lower part. Each follicle consists of an 

 adenoid rete, with very fine and regular meshes, filled with leuco- 

 cytes. The meshes are larger, and the leucocytes less crowded, at 

 the centre and periphery of the nodule, as shown in Fig. 248. At 

 the surface, where the follicle projects into the intestine, the villi 

 are usually absent, and the crypts of Lieberkuhn are found at its 

 circumference. One or more arterioles penetrate into the nodule, 



