ORGANS OF SENSE. 555 



blood of the inferior cava, yet is propelled in so great abundance to the 

 head and upper extremities, as to provide for the increased nutrition of 

 those important parts, and prepare them, by their greater size and deve- 

 lopment, for the functions which they are required to perform at the instant 

 of birth. 



4thly. That the blood circulating in the descending aorta is very impure, 

 being obtained principally from the returning current in the superior cava ; 

 a small quantity only being derived from the left ventricle. Yet is it from 

 this impure blood that the nutrition of the lower extremities is provided. 

 Hence we are not surprised at their insignificant development at birth ; 

 while we admire the providence of nature, that directs the nutrient current, 

 in abundance, to the organs of sense, prehension, and deglutition, organs 

 so necessary, even at the instant of birth, to the safety and welfare of the 

 creature. 



After birth, the foramen ovale becomes gradually closed by a membran- 

 ous layer, which is developed from the margins of the opening from below 

 upwards, and completely separates the two auricles. The situation of the 

 foramen is seen in the adult heart, upon the septum auricularum, and is 

 called the fossa ovalis ; the prominent margin of the opening is the annu- 

 lus ovalis. 



As soon as the lungs have become inflated by the first act of inspiration, 

 the blood of the pulmonary artery rushes through its right and left branches 

 into the lungs, to be returned to the left auricle by the pulmonary veins. 

 Thus the pulmonary circulation is established. Then the ductus arteriosus 

 contracts, and degenerates into an impervious fibrous cord, serving in 

 after life simply as a bond of union between the left pulmonary artery and 

 the concavity of the arch of the aorta. 



The current through the umbilical cord being arrested, the umbilical 

 arteries likewise contract and become impervious, and degenerate into the 

 umbilical ligaments of the bladder. 



The umbilical vein and ductus venosus, also deprived of their circulating 

 current, become reduced to fibrous cords, the former being the round liga- 

 ment of the liver, and the latter a fibrous band which may be traced along 

 the fissure for the ductus venosus to the inferior vena cava. 



NERVOUS SYSTEM. The brain is very soft, almost pulpy, and has a 

 reddish tint throughout ; its weight at birth, relatively to the entire body, 

 is as one to six, and the difference between the white and grey substance 

 is not well marked. The nerves are firm and well developed. 



ORGANS OF SENSE. 



Eye. The eyeballs are of large size and well developed at birth. The 

 pupil is closed by a vascular membrane called the membrana pupillaris, 

 which disappears at about the seventh month. Sometimes it remains per- 

 manently, and produces blindness. It consists of tw r o thin membranous 

 layers, between which the ciliary arteries are prolonged from the edge of 

 the iris, and form arches and loops by returning to it again, without anas- 

 tomosing with those of the opposite side. 



The removal of the membrane takes place by the contraction of these 

 arches and loops towards the edge of the pupil. The capsule of the lens 

 is extremely vascular. 



Ear. The ear is remarkable for its early development ; the labyrinth 



