300 COMPARATIVE ANATOMY 



placed opposite to one another. The arteries (and also certain of 

 the veins) divide up into smaller and smaller branches, eventually 

 giving rise to microscopic tubes called capillaries, the walls 

 of which consist merely of a single layer of epithelial cells, 

 and these again unite to form the factors of the veins. The walls 

 both of veins and arteries consist, in addition to the epithelium, of 

 connective and elastic tissue and of unstriated muscular fibres, and are 

 much thicker in the case of the arteries than in that of the veins, 

 in which the muscular elements may be altogether wanting. 



The nucleus of the red corpuscles persist, and the whole cell is biconvex, 

 in all Vertebrates below Mammals ; and, even in these, nucleated red cells 

 may be seen in the marrow of the bones, in the blood of the spleen, and 

 often in that of the portal vein. In all other parts of the body of Mammals 

 they lose their nuclei and become biconcave. In all Mammals, except the 

 Camelidse, the red corpuscles have the form of circular discs ; in the last- 

 mentioned group and in all other Vertebrates except Cyclostomes they are 

 oval. They are largest in certain Urodeles, being in Amphiuma as much 

 as 75/x in their longest diameter ; then come, in order, those of other 

 Urodeles and of Dipnoans, Reptiles, Anurans, Fishes, Birds, and Mammals, 

 in which latter order they are the smallest, varying in different families from 

 2'5/x (Tragulidre) to 10/*. 



The heart is enclosed within a serous membrane, the pericar- 

 dium (Fig. 240c), which consists of parietal and visceral layers ; 

 the former is invested by the mediastinum (p. 298), and the latter 

 is closely applied to the heart. Between the two layers is a space 

 filled with lymph, representing part of the ccelome ; this is usually 

 completely shut off from the abdominal cavity, but in Elasmo- 

 branchs the two communicate by means of pericardio-peritoneal 

 canals. 



The heart arises either as a single (Elasmobranchii, Amphibia) 

 or as a paired (Teleostei, Sauropsida, Mammalia) tubular cavity 

 in the splanchnic layer of the mesoblast (comp. note on p. 299) 

 along the ventral region of the throat, close behind the gill-clefts. 

 Its wall becomes differentiated into three layers, an outer serous 

 (pericardial), a middle muscular, and an inner epithelial. In this 

 respect it essentially corresponds with the larger vessels, in the walls 

 of which, as already mentioned, three layers can also be distinguished; 

 but in the heart the muscular fibres are striated. By a study of its 

 development we thus see that the heart corresponds essentially to 

 a strongly developed blood-vessel, which at first lies more or less in 

 the longitudinal axis of the body ; later, however, it becomes much 

 more complicated by the formation of various folds and swellings. 

 Thus the embryonic tubular heart becomes folded on itself and 

 divided into two chambers, an atrium or auricle and a ventricle 

 (Fig. 241). Between these, valvular structures arise, which only 

 allow the blood to flow in a definite direction on the contraction of 

 the walls of the heart, viz., from the atrium to the ventricle ; any 

 backward flow is thus prevented. The valves are formed by 



