130 



COMPARATIVE PHYSIOLOGY. 



Erection of the penis results from the conveyance of an ex- 

 cess of blood to the organ, owing to dilatation of its arteries, 

 and the retention of this blood within its caverns. 



Pig. 135.— Section of erectile tissue (Cadiat). a, trabecules of connective tissue, with 

 elastic fibers, and bundles of plain muscular tissue (c); 6. venous spaces (Scharer). 



The structure of the penis is peculiar, and, for the details of 

 the anatomy of both the male and female generative organs, 

 the student is referred to works on this subject ; suflBce it to 

 say that it consists of erectile tissue, the chief characteristic of 

 which is the opening of the capillaries into cavernous venous 

 spaces (sirmses) from which the veinlets arise ; with such an 

 arrangement the circulation must be very slow^the inilow 

 being greatly in excess of the outflow — apart altogether from 

 the compressive action of certain muscles connected with the 

 organ. The manner and duration of the act of copulation in 

 the domestic animals varies with the structure of the penis, the 

 animal's nervous excitability, etc. In the stallion the act is of 

 moderate duration, the penis long, and the glans penis highly 

 sensitive. 



In the bull the penis is of a different shape. During erec- 

 tion it is believed that the S-shaped curve disappears, and that 

 the extremity of the organ enters the mouth of the uterus itself. 

 Copulation is of very brief duration. 



In the dog the penis is of very peculiar formation. Its an- 



