593 



COMPARATIVE ANATOMY. 



thus, for example, the abdominal vein of the Salamander is enclosed 

 in a lymphatic vessel. 



There are, however, other vessels in the lower divisions besides 

 those which accompany the blood-vessels — in the skin, or on 



portions even of the digestive canal, or other 

 viscera. Peripherally, the lymphatic vessels 

 anastomose largely, and form capillary net- 

 works or similar spaces. These gradually give 

 off wider spaces, either in the form of canals, 

 or of sinuses with irregular boundaries, the 

 place of which is taken, in the higher Verte- 

 brata, by vessels allied in structure to veins. 



Although we may note that, as we pass 

 from the lower to the higher Vertebrata, there 

 is a gradual differentiation from spaces, similar 

 to those of the lacunar system of the Inverte- 

 brata, to a definitely developed canalicular 

 system, so that the interstitial nature of the 

 lymphatic ducts is well marked at the periphery 

 only; nevertheless, in the coeloni we have an 

 arrangement which indicates the origin of the 

 lymphatic vessels from a lower condition — for 

 the coelom is a lymphatic cavity. In this point 

 the coelom of the Vertebrata resembles closely 

 that of the Invertebrata. The communications 

 between the coelom and the pericardial cavity, 

 which obtain in various Fishes (Sturio, Selachii), are indications of 

 the same thing ; as are also the pleural cavities of the Mammalia, 

 which are merely differentiations of the general coelom. 



Fig. 3 10. Portion of tbo 

 Aortaof a Chelonian 

 (Chelydra) surrounded 

 by a lymphatic space, 

 a Aorta, b Outer wall 

 of the lymph sjmce, 

 which is removed at b' 

 so as to expose the blood- 

 vessel, c Trabeculae. 



§ 444. 



In Fishes the chief trunks have the form of lymph- sinuses. 

 There are generally two pairs of them, or one unpaired one is placed 

 below the vertebral column. The unpaired trunk divides into two 

 branches. Smaller sinuses, and narrower canals, form the lymphatic 

 vessels which are collected into these trunks. They are generally 

 connected with the venous system at two points. A cranial lym- 

 phatic sinus opens into the jugular vein of its own side, and two 

 sinuses, which receive lateral trunks, are connected by a trans- 

 verse anastomosis with the caudal vein near the last caudal 

 vertebra. 



The subvertebral lymphatic cavity of the Amphibia forms a portion 

 of the system which is about the same size as a very large subcutaneous 

 series of lymphatic cavities, which are present in these animals ; in 

 the anourous Amphibia especially, this latter series extends over a 

 large portion of the surface of the body. The lymphatic vessels of 

 the digestive canal (chyle-vessels) open into it, as do those of the 

 other viscera, while it is connected also with the lymphatics of the 



