THE REPRODUCTIVE ORGANS. 239 



supposed, during erection, to serve as a dam, preventing the entrance 

 of semen into the bladder. 



The ejaculatory ducts divide behind the prostate, one branch 

 forming the duct of the seminal vesicle, while the other becomes 

 continuous with the vas deferens. 



3. The Seminal Vesicles. — These are tubular sacs ending in blind 

 extremities, with occasional saccular branches given off from their 

 sides. They are lined with a mucous membrane covered with columnar 

 epithelium, resting upon areolar fibrous tissue. Outside of this is 

 a muscular coat containing internal circular and external longi- 

 tudinal fibres, and surrounded by an ill-defined fibrous coat that 

 passes into the general areolar tissue of the region. The seminal 

 vesicles sometimes contain semen, for which they may serve as a 

 temporary reservoir, but they also secrete a fluid that is mixed with 

 the semen at the time of ejaculation. 



4. The Vasa Deferentia. — The vas deferens of each side resembles 

 the seminal vesicle in structure. It is lined with columnar epi- 

 thelium, beneath which is a layer of areolar fibrous tissue, resting 

 upon the muscular coat. This is surrounded by fibrous tissue, 

 becoming areolar as it blends with that of the neighboring parts. 

 The muscular coat is thicker than that of the seminal vesicle, and 

 is divisible into an inner layer of circular and an outer layer of 

 longitudinal fibres. The mucous membrane, like that of the sem- 

 inal vesicle, is thrown into folds, which are longitudinal throughout 

 most of the course of the vas deferens, but are irregular in the 

 sacculated distal portions of the tube, giving the surface a reticu- 

 lated or alveolar appearance. 



5. The Epididymis. — The vas deferens of each side becomes con- 

 tinuous with the canal of the epididymis, which is an enormously 

 long tube, twenty feet, so convoluted and packed together as to 

 occupy but little space. It is lined throughout with columnar epi- 

 thelium, continuous with that of the vas deferens; but, except for a 

 short distance from the junction with the vas, the cells possess cilia 

 of considerable length, which induce currents toward the vas deferens. 

 The muscular coat of the latter is continued in the epididymis, but 

 is very thin. Opening into the canal of the epididymis are the vasa 

 efferentia of the testis. 



6. The Testis. — The testis is a compound tubular gland, of which 

 the secretion contains the spermatozoa. The latter are derived from 

 certain of the cells lining the tubules, and contain w r ithin their 



