ERECTION OF THE PENIS. 651 



occurs at this time becomes much larger, having a diameter of 

 from 12 to 20 mm. The two varieties are known respectively as 

 the corpus luteum spurium or of menstruation, and the corpus luteum 

 verum or of pregnancy. There is, however, no essential difference 

 between the two. It was supposed at one time that the existence 

 of pregnancy could be determined by the presence of a corpus 

 luteum in the ovary, but the existence of such bodies in undoubted 

 virgins has overthrown that theory absolutely. Is is said that in 

 the mouse there is no difference as to structure or size between 

 corpora lutea derived from follicles whose ova have been impreg- 

 nated and those whose ova have not been fertilized. 



Maturation of the Ovum. This process takes place in all ova, 

 and is necessary for their preparation for fertilization. In other 

 words, an immature ovum is not susceptible of being fertilized. 

 These changes take place while the Graafian follicle is also be- 

 coming mature, and are complete by the time the ovum is dis- 

 charged from the follicle. To understand the process thoroughly 

 one must be familiar with karyokinesis (p. 28). It may, how- 

 ever, here be briefly described as beginning with the migration of 

 the germinal vesicle to the periphery (Fig. 434), the rupture of 

 the nucleus, the formation of the spindle, etc., and the extrusion 

 of the polar bodies (Fig. 435) ; and thus is formed a new nucleus, 

 the female pronucleus (Fig. 436). If the ovum is unfertilized, it 

 undergoes disintegration, probably within eight days from the 

 time it left the ovary : but if fertilized, the female and male pro- 

 nuclei, the latter being derived from a spermatozoon, fuse and 

 form a new nucleus, the segmentation nucleus. 



Impregnation. In order that the ovum may be impregnated 

 or fertilized the spermatozoa must come in contact with it in the 

 generative passage of the female ; or, more properly speaking, one 

 spermatozoon must, for in the process of fertilization but one of 

 these structures is involved. This is preceded by erection of the 

 penis and ejaculation of the semen. 



Erection of the Penis. Any influence brought to bear upon 

 the sexual center, which is situated in the lumbar region of the 

 spinal cord, by which it is stimulated, results in the emission of 

 impulses through the nervi erigentes. This influence may come 

 from the brain in the form of mental impressions, or from stimu- 

 lation of the sensory nerve-endings in the penis. The efferent 

 impulses which reach the penis cause a relaxation of the muscular 

 structure of the trabecula?, thus increasing the capacity of their 

 interspaces, and also a dilatation of the arterial vessels, so that an 

 increased amount of blood is supplied to the organ. The veins 

 (Fig. 437) which return the blood from the penis are relatively 

 small, and are unable to return quickly the blood supplied by the 

 relaxed arteries ; this obstacle to the free return of the blood is 

 augmented by the compression of the veins produced by the con- 



