600 GENERATION AND DEVELOPMENT. 



form of the ventricle (see Fig. 233). The great curvature of the 

 horseshoe by the same means becomes much more developed 

 than the smaller curvature between the auricle and bulbus; 



FIG. 233. 



Heart of the chick at the 45th, 65th, and 85th hours, of incubation. 1, the venous 

 trunks; 2, the auricle; 3, the ventricle; 4, the bulbus arteriosus (after Dr. Allen 

 Thomson). 



and the two extremities, the auricle and bulb, approach each 

 other superiorly, so as to produce a greater resemblance 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 divi- 

 sions which are so early formed in the embryo ; but the auricle 

 is divided internally by a septum into a pulmonary and sys- 

 temic 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 aortse is not permanent, but becomes lost in the ven- 

 tricles. The septum dividing the ventricle commences at the 

 apex and extends upwards. When it is complete, a septum 

 is developed in the bulbus aortse, separating the roots of the 

 proper aorta and the pulmonary artery. The septum of the 

 auricles is developed from a semilunar fold, which extends 

 from above downwards. In man, the septum between the 

 ventricles, according to Meckel, begins to be formed about the 

 fourth week, and at the end of eight weeks is complete. The 

 septum of the auricles, in man and all animals which possess 

 it, remains imperfect throughout foetal life. When the par- 

 tition of the auricles is first commencing, the two venae cavse 

 have different relations to the two cavities. The superior cava 

 enters, as in the adult, into the right auricle ; but the inferior 

 cava is so placed that it appears to enter the left auricle ; and 

 the posterior part of the septum of the auricles is formed by 

 the Eustachian valve, which extends from the point of en- 



