918 



Fig. 445. 



DIAGRAM OF THE CIR- 

 CULATION ' AT THE 

 COMMENCEMENT OF 

 THE FORMATION OF 

 THE PLACENTA : 



SEEN FROM THE 

 FRONT. 



a, Venous sinus re- 

 ceiving all the sys- 

 temic veins ; 6, Right 

 auricle ; 6', Left 

 auricle ; c, Eight 

 ventricle ; c f , Left 

 ventricle; d, Bulbus 

 aorticus, subdividing 

 into, e, e', <?", Bran- 

 chial branches ; /,/, 

 Arterial trunks 

 formed by their con- 

 fluence ; g, g', Vena 

 azygos superior; h^ 

 A', Confluence of the 

 superior and inferior 

 azygos ; j, Vena cava 

 inferior ; k, k', Vena 

 azygos inferior; m, 

 Descending aorta; 

 n, n, Umbilical arte- 

 ries proceeding from 

 it; o, o', Umbilical 

 veins; g, Omphalo- 

 mesenteric vein ; r, 

 vesicle, t ; v, Ductus 



EMBRYOLOGY. 



by the ductus arteriosus. To sum up, the foetus never 

 receives pure arterial blood into its organs, this being 

 always mingled with venous blood, the mixture taking 

 place at several points : 1, By the foramen of Botal ; 2, 

 In the aorta by the ductus arteriosus ; 3, In the liver 

 by the ductus venosus. The head and neck are the 

 parts which receive the purest arterial blood, a fact 

 which explains the predominance of the anterior over 

 the posterior portion of the body of the foetus. 



At birth, the conditions of existence being suddenly 

 changed, marked modifications take place in the cir- 

 culatory apparatus. The lungs become the organs of 

 respiration, and rapidly increase in capacity ; the pul- 

 monary artery dilates to give passage to the blood that 

 flows to them ; while the ductus arteriosus is obliterated, 

 in order to isolate the arterial from the venous blood. 

 This separation of the two fluids also takes place in the 

 liver by the atrophy of the ductus venosus, and in the 

 heart by the occlusion of the foramen of Botal ; though, 

 according to M. Goubaux, that orifice frequently remains 

 open in young animals. Its persistence has also been 

 noted in the human adult. Notwithstanding the pre- 

 sence of this foramen, the circulation cannot be much 

 disturbed; as when the heart contracts, the auricles 

 become isolated by the constricting of the orifice and 

 the raising of a valve. 



DEVELOPMENT OF THE RESPIRATORY APPARATUS. 



Observers are not unanimous as to the develop- 

 ment of the lungs. According to Eeichert and Bischoff, 

 they arise from two small solid cellular masses lying on 

 the surface of the anterior portion of the intestinal 

 canal. These become channeled out into numerous 

 ramifying cavities (by the deliquescence or fusion of the 

 internal cells), which communicate with the trachea. 

 Costa states that they commence by a median, bud-like, 

 hollow process, that opens into the oesophagus. The 

 walls of the communicating aperture elongate consider- 

 ably, and at a later period form the trachea and larynx ; 

 while the hollow bud divides into two pyriform sacs, 

 each of which becomes broken up into a multitude of 

 subdivisions to constitute the pulmonary lobes, with their 

 vesicles and infundibula. 



The trachea is completed by the development of the 

 cartilaginous rings in the tube that binds the lungs to 

 the oesophagus. They appear at the commencement of 

 the third month. 



The larynx is developed in the same manner at the 

 pharyngeal opening; it is always somewhat undefined 



Omphalo-mesenteric artery distributed on the walls of the vitelline 

 venosus i y, Vitelline duct; z, Chorion, 



