Generation and Development 379 



the bladder and elongates the penis of the castrated animal, 

 no doubt produces what Colin has recorded of stallions. 



The influence of the cerebrum on the mechanism of 

 erection, viz., in stimulating the centre in the cord and 

 medulla, is well known. 



The first portion of the penis which receives the excess of 

 blood in erection is the corpus cavernosum ; the spongiosum 

 and glans are not fully erect in the stallion until the penis 

 is introduced into the vagina; at the moment of ejacu- 

 lation the glans swells enormously, apparently to cover 

 or grasp the os uteri. The blood sent to the penis for the 

 purpose of erection is practically imprisoned by the com- 

 pression exercised by the muscles of the perinseum on the 

 veins of the part, and this mechanism further maintains 

 the blood pressure. 



If the nerves of the penis be divided in the horse, erec- 

 tion is impossible, though desire may be intense. Gunther's 

 and Colin's experiments have placed this beyond doubt. 



Though the organ in the horse assumes such considerable 

 proportions, in the bull this is not so marked. The penis 

 in this animal comes to a narrow point without any of the 

 swelling observable in the stallion. In the ram, also, the 

 penis is narrow and pointed, and the peculiar vermiform 

 appendage at its extremity appears essential for successful 

 impregnation, for if removed the animal proves sterile. 



Sexual Intercourse is of short duration in the majority of 

 animals, excepting the dog and pig. Colin places it at 10 

 to 12 seconds for a vigorous stallion ; it is exceedingly rapid 

 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 act of coition, would rather point to the 

 fluid in this animal being directly passed, at any rate to 

 some extent, into the os uteri, and the pointed penis of the 

 bull and ram makes it nearly certain that much of the 

 fecundating fluid passes directly into the uterus. 



