INTRODUCTORY. 23 



circulation, but they anastomose less freely, tlieir valves 

 are less numerous, the fluid they contain is colourless, and 

 sooner or later almost all of them pass through certain 

 dark grey nodular bodies, found in various parts of 

 the body, termed lymphatic glands. These produce a 

 change in the nature of the contained fluid, and seem to 

 be mere means of economy of space, since some of the 

 lower animals in place of glands present extreme elonga- 

 tion and tortuosity of the lymph vessels. In these lower 

 aniniEfels the lymph vessels present in some parts dilata- 

 tions with thick muscular walls, lymph hearts, which 

 drive the lymph into the blood-vascular system in several 

 situations. In the higher vertebrata the communication 

 between the blood and lymph systems seems to occur only 

 at the anterior part of the thorax. Some authorities have 

 described these large lymph vessels as contractile. 



Lymphatic glands consist of a special investing coat 

 with trabeculse passing inwards, dividing the contained 

 space into cavities, of which the external are most marked, 

 all of which are lined with epithelium. The external por- 

 tion, therefore, is softest, and is termed the cortical portion ; 

 the firmer central structure is the medullary structure. A 

 number of beaded afferent lymphatics converge and pass 

 into a gland ; they seem primarily to pass to the medullary 

 portion in which they are arranged in a convoluted mass. 

 They are supposed to open into the cavities of the gland, 

 lymph spaces, and the lymph to pass irregularly through 

 these until it arrives at the commencement portion of the 

 efferent lymphatics, which, larger and less numerous than 

 the afferent vessels, on emerging from the gland pass 

 towards the main lymphatic trunks. 



The lymph vessels of the bowels are distinctively called 

 the lacteals, for the fluid they convey is rendered opaque 

 and milky in appearance by the presence of fatty globules 

 taken up from the contents of the intestines by these 

 vessels. They differ in no respect from other lymphatics ; 

 th,ey commence in the villi of the intestines either in blind 

 extremities, in loops, or in plexuses. 



