VASCULAR SYSTEM. 269 



region of the throat, close behind the gill-clefts. 1 Thus it is formed 

 from the same blastema as the muscular coat of the alimentary 

 canal, and its wall becomes differentiated into three layers, an 

 outer serous (pericardial), a middle muscular, and an inner 

 epithelial. In this it essentially corresponds in structure 

 with the larger vessels, in the walls of which three layers 

 can also be distinguished. 2 By a study of its development we 

 thus see that the heart represents essentially a strongly deve- 

 loped blood-vessel, which at first lies more or less in the longi- 

 tudinal axis of the body ; later, however, it becomes much more 

 complicated by the formation of various folds and swellings. In 

 this manner the folded tubular heart becomes divided into two 

 chambers, an atrium and a ventricle. 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. 



~~J3a. 



FIG. 214. DIAGRAM SHOWING THE PRIMITIVE RELATIONS OF THE DIFFERENT 



CHAMBERS OF THE HEART. 



Sv, sinus venosus, into which the veins from the body open ; A, atrium ; V, ventricle ; 

 Cn, conus arteriosus ; Ha, bulbus arteriosus. 



The valves are formed by a process of differentiation of the mus- 

 cular trabeculse of the walls of the heart, as will be explained later 

 on. The atrium, into which the blood enters, represents primi- 

 tively the venous portion of the heart, while the ventricle, from 

 which the blood flows out, corresponds to the arterial portion. The 

 venous end further becomes differentiated to form another chamber, 

 the sinus venosus (Fig. 214, Sv), and the arterial end gives rise 

 distally to a conus or truncus arteriosus; this is provided 

 with more or less numerous valves, and is continued forwards into 

 the arterial vessel, the enlarged base of which is spoken of as the 

 bulbus art eriosus (Ca, Ba). 



1 The primitive aortfe arise independently of the heart : they are formed by 

 peripheral (segmentally arranged ?) vascular processes, which pass towards the middle 

 line, and there turn forwards and backwards and become confluent, so as to form 

 longitudinal trunks. 



' The walls of the smallest blood-vessels, the capillaries, consist of a single 

 cellular layer, which corresponds to the inner epithelial layer (intima) of the larger 

 vessels. 



