122 ANIMAL PHYSIOLOGY. 
Erection of the penis results from the conveyance of an 
excess of blood to the organ, owing to dilation of ‘its arteries, 
and the retention of this blood within its caverns. 
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; suffice it to 
say that it consists of erectile tissue, the chief characteristic of 
which is the opening of the capillaries into cavernous venous 
spaces (sinuses) from which the veinlets arise; with such an 
arrangment the circulation must be very slow—the inflow 
being greatly in excess of the outflow—apart altogether from 
the compressive action of certain muscles connected with the 
organ. As previously explained, the spermatozoa originate in 
the seminal tubes, from which they find their way to the 
Fie. 140,—Section of parts of three seminiferous tubules of the rat (Schafer). @, with the 
spermatozoa least advanced in development ; b, more advanced ; ¢, containing fully de- 
veloped spermatozoa. Between the tubules are seen strands of interstitial cells, with 
blood-vessels and lymph-spaces. 
seminal vesicles or receptacles for semen till required to be 
discharged. The spermatozoa as they mature are forced on by 
fresh additions from behind and by the action of the ciliated 
cells of the epididymis, together with the wave-like (peristaltic) 
action of the vas deferens. Discharge of semen during coitus 
is effected by more vigorous peristaltic action of the vas defer- 
ens and the seminal vesicles, followed by a similar rhythmical 
action of the bulbo-cavernosus and ischio-cavernosus muscles, 
by which the fluid is forcibly ejaculated. 
Semen itself, though composed essentially of spermatozoa, 
