156 PHYSIOLOGY OF FARM ANIMALS [CH. 



which are present in most parts of the lining membrane of the 

 urethra also contribute to the production of semen. 



The Copulatory Organ. The penis is the intromittent organ 

 of copulation. Besides serving to conduct the urine to the exterior 

 through the channel of the urethra it has the further function of 

 conveying the semen into the genital passages of the female. This 

 latter function is made possible by the power of erection whereby 

 the organ can be inserted into the vagina of the female. 





8 / V't 







M 



"t, ^fl • ^. ■% '••.^■•'v ',-\-^'*.y- 





Fig. 77. Section through prostate (from Marshall), a alveolus lined by epithelium, 

 h concretion (often found in old subjects), c muscular fibres amid connec- 

 tive tissue, d blood vessel. 



The erectile tissue of the penis is contained chiefly in three 

 tracts, the two corpora cavernosa which are situated one on each 

 side and typically are united in the middle line, and the single 

 corpus spongiosum which is placed interiorly and surrounds the 

 urethral channel. The corjDora cavernosa are surrounded by an 

 integument containing connective tissue and unstriated muscle 

 fibres and giving off trabeculae which divide the structures into 

 blood spaces or sinuses. These spaces become engorged with 



