FUNCTIONS OF THE GENERATIVE ORGANS, ETC. 87 



to be composed of vesicles, or sacs, which contain a fluid, within 

 which are small bodies called ovula, and whicli are, indeed, the 

 germs of young animals. Whether the seminal fluid is conveyed 

 by the fallopian tubes to the ovaria, and there impregnates an 

 ovulum, as is supposed, or whether the vesicle bursts indepen- 

 dently of the copulative act, and is then conveyed to the uterus 

 there to be impregnated, or not, we cannot determine. 



The ovulum, being impregnated, is termed the ovum, and on 

 departing from the ovarium leaves a scar behind, which is called 

 a corpus luteum. 



The impregnated ovum, soon after its arrival in the uterus, 

 becomes firmly attached to this body, prior to which a quantity 

 of blood is determined to the uterus, by which a substance re- 

 sembling lymph is secreted, which furnishes not only a lining 

 membrane for the uterus, but also for the ovum itself. The 

 former is termed the deciduous, and the latter the reflected coat; 

 and each of them soon becoming abundantly furnished with blood 

 vessels, they together are called the Placenta, and the foetus is 

 thereby supplied with nourishment. The vessels of the mother, 

 and those of the foetus, do not communicate with each other by 

 any direct channel; but it is supposed that the foetal vessels 

 derive nourishment and purification by a sort of imbibition from 

 the mother's blood. 



The ovum, on leaving the ovaries, is furnished with a mem- 

 brane, called the Amnios, which afterwards increases in size, and 

 furnishes a liquid in which the foetus floats, comparatively free 

 from danger, and in all the ease and luxury of an hydrostatic bed. 

 It is supported in its situation by the umbilical cord, or navel 

 string, which is nearly three feet in length, and is composed of 

 the blood vessels, and a canal communicating with the bladder, 

 and proceeding from the navel of the foetus, and passing to the 

 deciduous membranes of the mare. This canal is called the 

 urachus, and serves to convey away the superfluous water that 

 the foetal bladder would be incapable of containing. 



The circulation of the blood in the foetus, it is evident, must 

 be very diflerent from that after birth. Nourishment is not 

 afforded by the stomach, bile is not secreted by the liver, nor is 

 the blood purified by the lungs : these organs are only supplied 

 with nourishment sufilcient for their own support and increase. 

 The arteries belonging to the maternal portion of the placenta 

 terminate partly in veins, and partly in exhalent surfaces : from 

 these exhalent surfaces, the minute branches of the umbilical 

 veins of the foetus absorb blood in a comparatively red and 

 purified state : they convey this blood into the foetal circulation 

 through the vein, which, in the animal after birth, is wholly en- 

 gaged in supplying the liver. It Avill thus be seen that the 

 umbilical veins convey red blood, and the umbilical arteries black 



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