FORMATION OF THE VALVES. 791 



be a peculiar twist of one or both, in order that they may finally unite 

 so as to become continuous. 



The completion of the partition of the aortic and pulmonary stems 

 is afterwards effected by the progress of the division from within out- 

 wards through the external walls of the tubes ; but the two vessels 

 remain united externally by a common envelope of pericardium. 



The remarkable cases sometimes observed of abnormal transposition 

 of the two great arterial stems from their natural connection with their 

 respective ventricles may be explained by reference to the history of the 

 development of the parts of the heart before given. 



Pormation of the Valves. — The formation of the auriculo-ventri- 

 cular and semilunar valves begins during the time of the changes 

 previously described by the projection of thick folds from the inner 

 wall of the heart. In the case of the semilunar valves the trifid divi- 

 sion is early perceived, but the cavities or sinuses within the valves 

 are late of being developed. In the auriculo-ventricular valves there 

 is at first an entire or annular projecting fold of the inner substance 

 round the orifice, and this becomes gradually divided into segments, 

 and the chordse tendincEe arej gradually produced by perforation of the 

 valve plate. (See Tonge in Proceed. Eoy. Soc, 1868.) 



The manner in which the pulmonary veins, which are formed 

 separately in the lungs, come to be connected with the left auricle has 

 not yet been ascertained. 



No further important changes occur in the internal structure of the 

 heart, but there are some w^hich affect the external form and thickness 

 of its walls. In early foetal Hfe the size of the heart bears a consider- 

 ably greater proportion to that of the body than at a later period. At 

 birth it is still proportionally large. For some time the auricular 

 portion remains more voluminous than the ventricular, but in the latter 

 half of foetal life the permanent proportion is more nearly established. 

 The walls of both ventricles are also thicker than in after life, and it is 

 especially deserving of notice that the wall of the right is up to near 

 the time of birth quite as thick as that of the left, — a peculiarity 

 which may be connected with the ofiice of the right ventricle to propel 

 the blood of the foetus through the extended course of the ductus arte- 

 riosus, the descending aorta and the placental circulation. 



DEVELOPMENT OF THE BLOOD-VESSELS. 



The Principal Arteries. The Aorta. — The most interesting part 

 of this history is that relating to the development of the aortaand the 

 larger vessels'^ arising from it. The double condition of the main trunk 

 of "the aorta has already been referred to as existing in the chick up to 

 near the end of the second day. About the fortieth hour the inedian 

 fusion or coalescence of the two vessels begins to take place in the 

 dorsal region, by their external union, at first in a very limited space, 

 and very soon afterwards by the formation of a perforation through 

 their united walls. The union of the two vessels which begins in the 

 dorsal region extends itself backwards towards the lumbar vertebrae, and 

 when it reaches the place where the omphalo-mesenteric arteries pass 

 out on each side, these vessels, each of which was originally the con- 

 tinuation merely of one of the aortas, appear now as branches of a single 

 and median aorta. The iliac vessels are the next large vessels formed 



