466 OF GENERATION AND DEVELOPMENT. 



of the body receives a mixture of this, with what has previously cir- 

 culated through the system. The Pulmonary arteries convey little 

 or no blood through the lungs ; the current of blood, propelled from the 

 right ventricle, passing directly onwards through the ductus arteriosus, 

 into the aorta. At birth, however, the course of the circulation under- 

 goes great changes, that it may be adapted to the new mode, in which 

 the infant is henceforth to obtain its nutrition and to tfarry on its respi- 

 ration. As soon as the lungs are distended by the first inspiration, a 

 portion of the blood of the pulmonary artery is diverted into them, and 

 there undergoes aeration ; and, as this proportion increases, with the 

 full activity of the lungs, the ductus arteriosus gradually shrinks, and its 

 cavity finally becomes obliterated. At the same time, the foramen ovale 

 is closed by a valvular fold ; and thus the direct communication between 

 the two auricles is cut off. When these changes have been accom- 

 plished, the circulation, which was before carried on upon the plan of 

 that of the higher Reptiles ( 563), becomes that of the complete warm- 

 blooded animal ; all the blood which has been returned in a venous state 

 to the right side of the heart, being transmitted through the lungs, be^ 

 fore it can reach the left side, or be propelled from its arterial trunks. 

 It is by no means unfrequent, however, for some arrest of development 

 to prevent the completion of these changes ; and various malformations, 

 involving an imperfect discharge of the circulating and respiratory 

 functions, may hence result. 



825. The average length of time, which elapses between Conception 

 and Parturition, in the Human female, appears to be 280 days, or 40 

 weeks. There can be little doubt, however, that Gestation, may be 

 occasionally prolonged for one, two, or even three weeks, beyond that 

 period ; such prolongation not being at all unfrequent amongst the lower 

 animals ; and numerous well-authenticated instances of it, in the Human 

 female, being upon record. Upon what circumstances this departure 

 from the usual rule is dependent, has not yet been ascertained ; but it 

 is a remarkable circumstance, ascertained by the observations of cattle- 

 breeders, that the male has an influence upon the length of gestation, 

 a large proportion of cows in calf by certain bulls exceeding the usual 

 period, and a small proportion falling short of it. In such cases, we 

 must attribute the prolongation of the. period to some peculiarity in the 

 embryo, derived from its male parent. 



826. The shortest period at which Gestation may terminate, consis- 

 tently with the life of the child, has not yet been precisely ascertained ; 

 the difficulty of determining the precise date of conception being usually 

 such, in this case as in the preceding, as to prevent the exact length 

 of the Gestation from being known. Thus, the commencement of preg- 

 nancy ^ being fixed by the time of the cessation of the Catamenia, when 

 there is no more definite guide, it is obvious that the act of Conception 

 may have taken place during any part of the interval that has elapsed 

 since the last monthly period ; and thus a doubt may exist as to the 

 length of the Gestation, to the extent of, from one to three weeks. There 

 are very satisfactory cases on record, in which, from the degree of de- 

 velopment of the infant at birth, as well as from other circumstances, 

 it might be certainly known not to have attained 26 or 27 weeks, or 



