280 HAND-BOOK OF PHYSIOLOGY. 



The bulging out of the middle portion inferiorly gives the first indication 

 of the future form of the ventricle (Fig. 438). The great curvature of 

 the horseshoe by the same means becomes much more developed than the 

 smaller curvature between the auricle and bulbus; and the two extremi- 

 ties, the auricle and bulb, approach each other superiorly, so as to produce 

 a greater resemblance to the later 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 pulmo- 

 nary and systemic auricle. In Eeptiles, not merely the auricle is thus 

 divided into two cavities, but a similar septum is more or less developed 



FIG. 438. 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. (Allen Thomson.) 



in the ventricle. In Birds and Mammals, both auricle and ventricle 

 undergo complete division by septa; whilst in these animals as well as in 

 reptiles, the bulbus aortas is not permanent, but becomes lost in the ven- 

 tricles. The septum dividing the ventricle commences at the apex and 

 extends upward. The subdivision of the auricles is very early fore- 

 shadowed by the outgrowth of the two auricular appendages, which occurs 

 before any septum is formed externally. The septum of the auricles is 

 developed from a semilunar fold, which extends from above downward. 

 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 partition 

 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 auri- 

 cles is formed by the Eustachian valve, which extends from the point of 

 entrance of the inferior cava. Subsequently, however, the septum, grow- 

 ing from the anterior wall close to the upper end of the ventricular sep- 

 tum, becomes directed more and more to the left of the vena cava inferior. 

 During the entire period of foetal life, there remains an opening in the 

 septum, which the valve of the foramen ovale, developed in the third 

 month, imperfectly closes. 



