REPR OD UCTION. 449 



and is covered by a layer of muscular fibres constituting a distinct muscle — the 

 bulbs of the corpora cavernosa by the ischio-cavernosi (erectores penis), that of 

 the corpus spongiosum (called bulbus urethne\ by the bulbo-cavcrnosu.s (accel- 

 erator urinai). At its distal end each corpus cavernosum terminates bluntly, 

 while the corpus spongiosum projects farther and enlarges to form the extrem- 

 ity of the organ, the glans penis. Each corpus is spongy in consistence, being 

 formed of a trabecular framework of white and elastic connective tissue and 

 plain muscular fibres, with cavernous venous spaces, and it is covered by a 

 tough fibrous tunic. When the spaces are distended with blood the whole 

 organ becomes hard, rigid, and erect in position. The mechanism of erection 

 will be studied more in detail later (p. 463). The penis, especially toward 

 its termination, is beset with end-bulbs, Pacinian bodies, and other nerve-ter- 

 minations, which make it particularly sensitive to external stimulation. 



C. The Female Reproductive Organs. 



The female reproductive organs, already mentioned, have as their specific- 

 functions the production of the essential female germ-cells, the ova, and their 

 transference to the uterus, and, if unfertilized, to the outside world ; if the 

 ova are fertilized, other specific functions are the protection and nutrition of 

 the developing embryo, its ultimate transference to the outside world, and 

 the nutrition of the child during early infancy. 



The Ovum. — The human ovum was discovered in 1827 by von Baer, and 

 it was he who first completely traced the connection between ova in the gene- 

 rative passages and ova in the Graafian 

 follicles of the ovary. The conception 

 of ova as the essential female element 

 had, however, long been held, and Har- 

 vey's dictum of the seventeenth century, / 

 that everything living is derived from 

 an egg (omne vivum ex ovo), is well 

 known. The human ovum, as it comes 



from the ovary, is a spherical, proto- 

 plasmic cell (Fig. 220), averaging with 

 the zona radiata, approximately 0.2 milli- 

 meter (jyt inch) in diameter. As in 

 other cells, the cell-body may be distin- 

 guished from the nucleus, the proto- Fig. 220,-HTuaan ovum (modified from Na- 

 o ' tr gel): », nucleus (germinal vesicle) containing 



plasm of the former being called cyto- the amoeboid nucleolus (germinal spot) ; d,deu- 

 7 t •, n , ,i , toplasmic zone; />. protoplasmic zone; c, zona 



plasm. In its finer structure the cyto- riuiiatu; v I , ( ,, vit( . lliu , sl , a( . ( , 



plasm consists of an excessively delicate 



network of protoplasmic substance. As in other mammalian eggs, it proba- 

 bly contains, adjoining the nucleus, a minute, specially differentiated portion, 

 consisting of a single or double centrosome surrounded by an attraction sphere 

 (Fig. 221, .4). For some distance inward from the border the cytoplasm is 

 pure and transparent, and this portion is often called the protoplasmic zone 

 Vol. IT.— 29 



