REPRODUCTION. 651 



ing through the erectile masses relaxes. As a result the whole 

 organ becomes distended and finally rigid and erect. The 

 co-ordinating centre of erection lies in the lumbar region of the 

 spinal cord, and may be excited reflexly by mechanical stimu- 

 lation of the penis, or under the influence of nervous impulses 

 originating in the brain and associated with sexual emotions. 

 The corpus spongiosum resembles the corpora cavernosa in 

 essential structure and function. 



The skin of the penis is thin and forms a simple layer for 

 some distance; towards the end of the organ it separates and 

 forms a fold, the foreskin or prepuce, which doubles back, 

 and, becoming soft, moist, red, and very vascular, covers the 

 glands to the meatus urinarius, where it becomes continuous 

 with the mucous membrane of the urethra; in it, near the 

 projecting posterior rim of the glans, are imbedded many 

 sebaceous glands. It possesses nerve end organs (genital 

 corpuscles) which much resemble end bulbs in structure. 



The Seminal Fluid. The essential elements of the tes- 

 ticular secretion are much modified cells, the spermatozoa, 

 which are passed out with some albuminous liquid. The 

 spermatozoa (Fig. 186) are motile bodies about 

 0.04 m.m. (-g-J^ inch) in length. They have a 

 flattened clear body or head and a long vibratile 

 tail or cilium ; the portion of the tail nearest to 

 the head is thicker than the rest, and is known 

 as the neck. The mode of development of a 

 spermatozoon shows that the head is a cell-nu- 

 cleus and the neck and tail a modified cell- 



hnrlv ' n s ' tle view, a, 



DC1 y- head; 6, neck; c. 



On cross-section a seminiferous tubule pre- tail - 

 sents externally a well-marked basement membrane, upon 

 which are borne several layers of cells; the lumen or bore of 

 the tubule is in great part occupied by the tails of sper- 

 matozoa projecting from some of the lining cells. The outer 

 cells, those next the basement membrane, are arranged in a 

 single layer, and are usually found in one or other stage of 

 active karyokinetic division (p. 19). The result of the divi- 

 sion is an outer cell, which remains next the basement mem- 

 brane to repeat the process, and an inner, which is the mother - 

 cell of spermatozoa. The latter cell by repeated mitotic divi- 

 sion give rise to a number of cells lying side by side and each 

 having a relatively large nucleus and small cell-body. These 



