506 



PHYSIOLOGY 



CHAP. 



cavities, such as the peritoneum, the pleura, the pericardium, the 

 meninges, and, generally speaking, the whole of the serous sheaths 

 that invest the organs, form part of the lymphatic system in the 

 widest sense of the word. 



As regards the morphological questions, many of them very 

 complicated and much disputed, that arise over this system, we 

 must refer to treatises on anatomy and histology. Here we can 

 only summarise the general notions that are most intimately 

 connected with physiological problems. 



From the structural point of view, we may distinguish in 

 the lymphatic system : the lymph vessels properly so-called, the 

 lymphatic capillaries, the parenchymatous lymph spaces, the large 

 lymphatic or serous cavities. 



(a) The lymphatic vessels constitute a rich system of canals 



FIG. 238. Supra valvular swelling of. lymphatic in cat's mesentery, treated with silver nitrate. 

 (Ranvier.) The smooth muscular fibres which surround the vessel interlace in various 

 directions at the seat of the swelling. 



which are very similar in the structure of their walls to the veins, 

 and like them are richly provided with valves, which open centri- 

 petally and close in a centrifugal direction. These are specially 

 abundant in the small lymphatic vessels, e.g. in those of the 

 mesentery. Above each valve the vessel is somewhat dilated, so 

 that when there are many valves the vessel assumes a moniliform 

 or beaded appearance. The muscle cells in the tunica media of 

 the vessel walls are for the most part arranged in a circular 

 direction ; but at the supra valvular points of dilation they run in 

 various directions, so as to form a network (Fig. 238). Like the 

 blood-vessels, the lymphatics lie in a bed of connective tissue, and 

 gradually unite into vessels which become increasingly larger, until 

 they finally converge (in man and in the higher vertebrates) into 

 two principal channels: the thoracic duct, which opens by an 

 orifice provided with valves into the left subclavian vein ; and the 

 right lymphatic trunk, which opens into the right subclavian vein. 

 The lymphatics from the right side of the head and neck, right arm 



