FCETAL CIRCULATION. 



755 



Membrana pupillaris, Human foetus, cv". 



viclual development attained at birth parallel, in Mammals, those 

 in Birds expressed by the terms altrices and prcecoces (vol. ii. 

 p. 265). The hoofed quadruped enters the world with the use 

 of all its senses ; in a few hours can follow the dam, and keep 

 pace with her if she sees 

 cause for flight : the feline 587 



is born blind and helpless ; 

 some days elapse ere the 

 commissure of the eyelids is 

 unsealed. Corresponding 

 steps in the human organ 

 of vision are completed be- 

 fore birth. At the fifth 

 month of foetal life the pu- 

 pillary aperture is very 

 wide, and is occupied by a 

 rich layer of looped capilla- 

 ries supported by a produc- 

 tion of the membrane of the 

 aqueous humour, fig. 587, A. 

 As the iris is developed the 



pupil contracts and the vessels of the pupillary membrane diminish 

 in size and number ; so that at the eighth month only a few vessels 

 are seen crossing the transparent membrane, as at B. Shortly 

 before birth, or for a week after, a mere shred of the membrane 

 may be detected, as in c and D, and these are soon absorbed. 



§ 410. Foetal circulation. — The early stages in the development 

 of the vascular system closely correspond, in Mammals, with those 

 in Birds (vol. ii. p. 263, fig. 136): the steps in the establishment 

 of the aortic arch, with their relations to conditions of primary 

 branches characteristic of species, and to rare anomalies, have been 

 explained at pp. 534-537; here, therefore, there only remain a few 

 words to be said of the foetal characters of the circulating system. 



The blood of the foetus, after passing through the ramifications 

 of the allantoic arteries, fig. 588, u', u, in the placenta, returns 

 by the allantoic vein, u. This, on entering the abdomen, passes 

 above and superficial to the duodenum, within the peritoneal fold 

 called ( suspensory ligament ' of the liver, to the great fissure of 

 that organ, where it carries part of its blood directly, by the 

 ' ductus venosus,' d, to the post-caval, v, and part is distributed by 

 the branches of the portal vein, L, through the substance of the 

 liver, and is then conveyed by the hepatic veins, I, into the 

 general current of the returning blood. Thus, the right auricle 



3 c 2 



