VASCULAR SYSTEM 395 



serous (pericardium), a middle muscular (myocardium), and an 

 inner epithelial (endocardium) . l 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, and the boundaries between the 

 cells of which they are primarily composed may disappear, so that a 

 syncytium results. 



By a study of its development we thus see that the heart 

 corresponds essentially to a strongly-developed blood-vessel, which 

 later becomes complicated by the formation of various folds and 

 swellings. The embryonic tubular heart, 

 which contracts peristaltically, undergoes a 

 division into two chambers, an atrium or 

 auricle, and a^ ventricle, between which 

 valvular structures arise from the endo- 

 cardial layer: these only allow the blood 

 to flow in a definite direction on the con- 

 traction of the walls of the heart, viz., 

 from the atrium to the ventricle, and any 

 backward flow is thus prevented (Fig. 298). 



The atrium, into which the blood enters, 

 represents primitively the venous portion 

 of the heart, the ventricle, from which p IG . 298. DIAGRAM SHOW- 

 the blood flows out, corresponding to the ING THE PRIMITIVE RE- 

 arterial portion. The venous end further B ^^ 

 becomes differentiated to form another HEART. 



chamber, the sinus venosus, which opens 



. . , A, atrium or auricle; Ba, 



into the atrium by a narrow aperture bulbus and Ca conus ar . 



provided with two valves ; while the teriosus (together consti- 



arterial end gives rise distally to a truncus tutin S the .embryonic 

 , ^ i i r> truncus artenosus) ; ov, 



arteriosus ; the proximal muscular end ot sinus ven osus, into which 

 this (coii us arteriosus or pylangium) is pro- the veins from the body 

 vided with more or less numerous valves P en : v > ventricle, 

 arranged in longitudinal rows, and its distal 



end (bulbus arteriosus or synangium) is continued forwards into 

 the arterial vessel (ventral aorta). 



These four chambers of the heart now contract rhythmically 

 in the following order: sinus venosus, atrium, ventricle, conus 

 artenosus. The bulbus, which may be more or less swollen and 

 corresponds to the base of the ventral aorta, contains no striated 

 muscular fibres. 



The ventral aorta gives off right and left a series of symmetrical 

 afferent branchial arteries (Figs. 299-301, 320 and 321), each of 

 which runs between two consecutive gill-clefts, branches out into 

 capillaries in the gills, when present, and then becomes continuous 

 with a corresponding efferent branchial artery. After the first pair 



1 The epithelial lining of the heart is said by some embryologists to be derived 

 from the endoderm, by others from mesenchymatous cells. 



