696 A MANUAL OF VETERINARY PHYSIOLOGY 



in the majority of animals, excepting the dog, pig, and camel. 

 Colin places it at ten to twelve seconds for a vigorous stallion. 

 It is exceedingly rapid, almost instantaneous, in the bull and 

 ram, probably from the peculiar shape of their intromittent 

 organ. The spermatic fluid is forced into the vagina, or even 

 directly into the uterus. The peculiar termination of the urethra 

 of the horse, and the bulbous enlargement of the glans during 

 the final act of coition, point to the organ grasping the os at the 

 moment of ejaculation, while the projecting portion of the 

 urethra is inserted into it, by which means some of the fluid is 

 undoubtedly directly injected into the uterus ; the pointed penis 

 of the bull and ram makes it certain that such is also the case 

 in these animals. An examination of the uterus of the sheep 

 and dog a few minutes after coition has revealed the presence 

 within it of spermatozoa. There is ample evidence that the 

 spermatozoa may remain alive for several days within the uterus. 

 At the moment of intercourse the uterus becomes erect, and the 

 introduction of the male element into it is further assisted by 

 the aspiration following its subsidence. The actual mechanism 

 of ejaculation is produced by a contraction of the vesiculae 

 seminales, the prostate gland, and probably of the vasa deferentia, 

 through the reflex action of the ejaculation centre in the lumbar 

 and sacral portions of the cord. By this means the seminal 

 fluid is forced out of the vesiculae into the urethra, and by means 

 of the muscles of the perinaeum is forcibly ejected from the 

 urethra. In animals possessing no vesiculae, such as the dog, 

 ejaculation takes place direct from the testicle and vas deferens. 

 The Ovum.- — The main function of the ovary is to contain the 

 ova and favour their development. It does not secrete them, 

 the ova are laid down with the other cells of the body very early 

 in the life of the embryo, a portion of material being set aside 

 for their special development, as apart from the cells required for 

 the construction of the other portions of the body. It is a very 

 remarkable fact that within a few hours of the impregnation 

 of the ovum, one of the first acts in the cellular scheme is to 

 provide for the reproduction of the future animal, the prospective 

 parent of which at this stage is a* mere mulberry-like mass of cells. 

 The germinal cells, as these are called, as distinct from the body 

 or somatic cells, subsequently form an epithelial layer, which 

 grows into the body of the future ovary as a long cylinder of 

 cells ; these are eventually cut off, and remain in the ovarian 

 structure. At birth the ovaries contain some thousands of eggs, 

 some of which in due course, as the period of puberty arrives, 

 take on further and more active changes. At this time the 

 most advanced cells are enclosed in a follicle containing fluid, 

 the Graafian follicle. Lining the follicle is a layer of cells known 



