DEVELOPMENT. 689 



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 but incomplete is more or less developed 

 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 aortae is not permanent, but becomes lost in the ven- 

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

 extends upwards. 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 auri- 

 cles is developed from a semilunar fold, which extends from above down- 

 wards. 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 cavae 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 entrance of the inferior cava. Subsequently, however, the 

 septum, growing from the anterior wall close to the upper end of the 

 ventricular septum, becomes directed more and more to the left of the 

 vena cava inferior. During the entire period of fcetal life, there remains 

 an opening in the septum, which the valve of the foramen ovale, devel- 

 oped in the third month, imperfectly closes. 



The bulbus arteriosus, which is originally a single tube, becomes 

 gradually divided into two by the growth of an internal septum, which 

 springs from the posterior wall, and extends forwards towards the front 

 wall and downwards towards the ventricles. This partition takes a some- 

 what spiral direction, so that the two tubes (aorta and pulmonary artery) 

 which result from its completion, do not run side by side, but are twisted 

 round each other. 



As the septum grows down towards the ventricles, it meets and coa- 

 lesces with the upwardly growing ventricular septum, and thus from the 

 right and left ventricles, which are now completely separate, arise respec- 

 tively the pulmonary artery and aorta, which are also quite distinct. The 

 auriculo-ventricular and semilunar valves are formed by the growth of 

 folds of the endocardium. 



At its first appearance, as we have seen, the heart is placed just be- 

 neath the head of the foetus, and is very large relatively to the whole 

 body; but with the growth of the neck it becomes further and further 

 removed from the head, and is lodged in the cavity of the thorax. 



Up to a certain period the auricular is larger than the ventricular 

 44 



