432 COMPARATIVE ANATOMY 



veins only, it is called a rete mirabile simplex : if of a combination 

 of both kinds of vessels, it is known as a rete mirabile duplex. 



The retia mirabilia serve to retard the flow of blood, and thus 

 cause a change in the conditions of diffusion. They are extremely 

 numerous throughout the Vertebrate series, and are found in the 

 most varied regions of the body, as, for instance, in the kidneys 

 (glomeruli) where their above-mentioned function is most clearly 

 seen ; on the ophthalmic branches of the internal carotid ; on the 

 pseudobranchs and on the vessels of the swim-bladder in Fishes ; on 

 the portal vein ; and along the caudal portion of the vertebral 

 column in Lizards. In Mammals, they are best developed in 

 Edentates, but occur also in Cetacea, Pinnipedia, Rodents, 

 Marsupials, Lemurs, &e. 



LYMPHATIC SYSTEM. 



The lymphatic system consists of branched lymph-vessels, 

 situated in the connective tissue of various parts of the body, and 

 of lympli-sinuses : it serves to collect the blood-plasma which has 

 passed through the walls of the capillaries and to pass it into the 

 veins. The lymph, as already mentioned, contains leucocytes. 



It is probable that in all Craniates the lymphatic system is 

 primarily paired and symmetrical : the first trunks which appear 

 in the embryo have apparently a similar course to that of the main 

 veins. Thus paired vessels (cephalic ducts) extend backwards ^rom 

 the head, and others (thoracic ducts) forwards along the trunk in 

 similar positions to the anterior and posterior cardinal veins 

 respectively. On either side the cephalic and thoracic ducts unite 

 and open into a vein in the region of the precaval. In the course 

 of development, however, anastomoses are formed between the 

 trunk of either side : thus portions of the main trunks may 

 become of minor importance and undergo reduction, so that 

 asymmetry results, as is sometimes also the case in the venous 

 system. Lymph-vessels are usually abundant in various parts 

 (e.g. under the skin, on the alimentary canal) and may form 

 complicated networks. 



In Fishes, the lymph-vessels are in many respects not so 

 plainly differentiated from the venous system as in higher forms ; 

 a lymph-sinus connected with a vein occurs on either side in the 

 scapular region, and into it lymphatic trunks from the head and 

 body open. 1 A large vessel extends along the spinal canal, and 

 a subvertebral lymph-sinus surrounds the aorta and communicates 

 with others in the mesentery which receive the vessels from the 

 abdominal viscera. Lymphatics are also present on the walls of 

 the heart and great blood-vessels, forming perivascular sheaths. 



1 The lacunar spaces in Cyclostomes, generally regarded as belonging to the 

 lymphatic system, have been found to contain bloo'd. 



