766 GENERATION AND DEVELOPMENT. 



most anterior the bulbus arteriosus. These three parts of 

 the heart contract in succession. The auricle and the 

 bulbus arteriosus at this period lie at the extremities of the 

 horse- shoe. The bulging out of the middle portion in- 

 feriorly gives the first indication of the future form of the 

 ventricle (see fig. 233). The great curvature of the horse- 



Fig. 233-* 



shoe by the same means becomes much more developed 

 than the smaller curvature between the auricle and bulbus ; 

 and the two extremities, the auricle and bulb, approach 

 each other superiorly, so as to produce a greater resem- 

 blance to the latter form of the heart, whilst the ventricle 

 becomes more and more developed inferiorly. The heart 

 of fishes retains these three cavities, no further division by 

 internal septa into right and left chambers taking place. 

 In Amphibia, also, the heart throughout life consists of the 

 three muscular divisions which are so early formed in the 

 embryo ; but the auricle is divided internally by a septum 

 into a pulmonary and systemic auricle. In reptiles, not 

 merely the auricle is thus divided into two cavities, but a 

 similar septum is more or less developed in the ventricle. 

 In birds, mammals, and the human subject, both auricle 

 and ventricle undergo complete division by septa ; whilst 

 in these animals, as well as in reptiles, the bulbus aorta3 is 

 not permanent, but becomes lost in the ventricles. The 

 septum dividing the ventricle commences at the apex and 

 extends upwards. When it is complete, a septum is 

 developed in the bulbus aorta3, separating the roots of the 



* Fig. 233. Heart of the chick at the 45th, 65th, and 85th hours of 

 incubation. I, the venous trunks ; 2, the auricle ; 3, the ventricle ; 

 4, the bulbus arteriosus (after Dr. Allen Thomson). 



