REPRODUCTION 309 



vasa deferentia, where it is stored up. During the ejacula- 

 tion, it is mixed with the secretion of the vesicular seminales, 

 the prostate, and Cowper's glands. Very little is known 

 concerning the secretions of these glands or the nature and 

 importance of their secretions. The prostate is supposed to 

 secrete the spermin and odoriferous substance found in the 

 mixed seminal fluid. 



3. Discharge of the seminal fluid. Ejaculation. The 

 discharge of the seminal fluid takes place during erection by 

 the activity of certain muscles by which the fluid is forced 

 out of the vasa deferentia and the urethra. 



(a) Erection. During erection the blood vessels of the 

 penis are well filled. This filling of the penis is brought 

 about by 



(1) Increase .of blood carried to it by the dilation of the 

 arteries. The dilation is produced by the vaso-dilator 

 nerves, the nervi erigentes (see page 77). 



(2) The removal of the blood is prevented by the com- 

 pression of the venae profundae penis. The compression of 

 these veins is produced by the contraction of the musculus 

 trans versus perinei. 



The centre by which the erection is brought about is sit- 

 uated in the lumbar cord. It can be stimulated either 

 reflexly by the stimulation of the sensory nerves of the penis 

 or by impulses from the cerebral hemispheres (psychical). 



(b) Ejaculation. The ejaculation is accomplished by the 

 peristaltic contractions of the muscles of the vasa deferentia 

 and vesicular seminales which drive the fluid into the urethra 

 and thence it is propelled forward by the contraction of the 

 bulbo- and ischio-cavernosus muscles. The passage of the 

 urinary bladder is cut off by the erection of the caput 

 gallinaginis. The act of ejaculation can be called forth 

 reflexly by stimulating the sensory nerves of the penis. 

 The centre of ejaculation lies in the lumbar cord. 



The amount of seminal fluid discharged during one 

 ejaculation is I 6 cc. 



