ORGANS OF THE BODY. 555 



The proportion of casein in woman's milk is, according to Simon, about 

 3 '5 per cent, on an average ; that of fats, 2*5-4 per cent. ; of sugar of milk, 

 between 4 and 6 per cent. ; of salts (among which phosphatic earths pre- 

 dominate), 0-16-0-20 per cent. 



The average amount of milk secreted daily by the human female, 

 during the period of lactation, is somewhat over 1000 grammes. About 

 50 or 60 grammes may be produced by one breast in two hours (Lam- 

 perierre). 



The use of milk is, as is well known, for the aliment of the infant. It 

 is secreted at the expense of the nutritive material of the mother's blood, 

 and may be designated as the prototype of all aliment. 



If we compare the ingredients of milk with those of the plasma of the 

 blood (p. 115), we find that the mineral constituents of the latter may 

 have simply transuded into the former, somewhat in the same manner as 

 that in which they find their way into the urine. But the three series of 

 organic substances are not to be found as such in the blood, or, if so, 

 only in small amount. To the first of these, casein and sugar of milk 

 belong, the sources of which may be regarded as albumen and grape sugar; 

 to the third the fatty matters. All this seems to indicate an inherent 

 power in the mammary gland of causing a species of fermentation, as also 

 of producing within its cells a part, at least, of the oily matters found in 

 the milk. 



The mode in which the secretion of the mammary gland is produced in 

 the interior of the vesicles is similar to that in which the sebaceous 

 matter of the skin is formed. The gland-cells become enlarged by the 

 generation within them of oil globules (fig. 539, 2, b), and are in this way 

 physiologically destroyed, at least in many cases, although the membrane- 

 less body of the contractile gland-cell no doubt frequently enough 

 simply disgorges its fatty contents. During the less active formation of 

 the colostrum these cells, or fragments of them, are carried off in the 

 watery portion of the milk. The gland-cell of the suckling woman is 

 regarded by us as a very transitory structure. 



283. 



The male generative apparatus consists of two testicles, enclosed in the 

 scrotum, and invested with their several tunics ; of the excretory $ucts, 

 emptying themselves into the urethra; of the copulative organ; and, 

 finally, of accessory structures. Among the latter we have the single 

 prostatic gland, a pair of glands known as Cowper's, and the ves-iculcR 

 seminales. 



The testis, with its accessory epididymis, is a gland consisting of a 

 multitude of fine and very tortuous tubules, known as the tubuli semi- 

 niferi. The whole is covered by a fibrous investment, to which the name 

 of tunica albuginea, s. propria test'is (p. 227), has been given, a tough, 

 whitish membrane of considerable thickness. It is again contained within 

 another sac, the tunica vaginalis propria, a serous investment, whose 

 internal portion (t. adnata) cannot be distinguished from the albuginea. 

 Finally, the testicle and spermatic cord are enveloped in the t. vaginalis 

 communis, a strong bag, composed of a serous and fibrous portion, which 

 contains, around its junction with the vaginalis propria and epididymis, a 

 number of contractile fibre-cells (Kodliker). Upon this coat the striped 

 fibres of the cremaster muscles are situated externally. This vaginalis 

 communis is connected without with the muscular tunic of the scrotum, 



