8 THE HUMAN BODY. 



erect. The co-ordinating centre of erection lies in the lumbar 

 region of the spinal cord, and may be excited reflexly by 

 mechanical stimulation 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, 

 j 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 prepuce, which doubles back, and, 

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

 glans to the meatus urinarius, where it becomes continu- 

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

 the projecting posterior rim of the glans, are imbedded 

 many sebaceous glands. 



Histology of the Testis and its Secretion. Each semi- 

 niferous tubule consists of a basement membrane, sup- 

 porting an epithelium, which in early life forms a definite 

 lining to the tube; in the adult the cells multiply so as to 

 nearly fill the cavity. From these cells the spermatozoa 

 are formed. The seminal fluid is an albuminous liquid con- 

 taining granules and spermatozoa; exam- 

 ined with the microscope the latter are seen 

 as rapidly moving minute objects (Fig. 161) 

 each consisting of a broad flattened body 

 and a slender vibratile tail or cilium; they 

 are about 0.04 mm. (-y-J-y inch) in length. 

 The seminal fluid is not fully formed when 

 it leaves the seminiferous tubules; com- 

 ' magni- pletely developed spermatozoa are first 

 fied 350 diameters, found in the vasa recta, and even in the 



a, viewed from the 



side; 6, seen from contents of the vas deferens cells are often 

 found which have not yet finished the pro- 

 duction of a spermatozoon; probably, therefore, the fertil- 

 izing liquid is first fully elaborated near the junction of the 

 ras deferens with the duct of the seminal vesicle. 



The Reproductive Organs of the Female. Each ovary 

 (0, Fig. 162) is a dense oval mass about 3.25 cm. (1.5 

 inches) in length, 2 cm. (0.75 inch) in width and 1.27 cm. 

 (0.5 inch) in thickness; it weighs from 4 to 7 grams 



