678 THE PHYSIOLOGY OF REPRODUCTION 



94, 96, and even 103 in whose semen active sperms were found.^ 

 There can be no doubt, however, that the spermatozoa are 

 produced in far less abundance in old age. 



In women the period of senescence is usually reckoned from 

 the menopause. 



It is difficult to form any accurate comparison between the 

 phases of life of men and those of animals, partly because so 



Fig. 152. — Group of nerve cells from the first cervical ganglion of a man 

 of ninety-two. (After Hodge, from Minot's Age, Growth, and Death, 

 G. S. Putnam & Sons, and John Murray.) 



C, C, cells still intact, but shrunken and loaded with pigment ; 

 c, c, cells which have disintegrated. 



little is known regarding the conditions of natural senescence 

 and death in animals. Smith ^ remarks that few horses live 

 long enough to show much sign of arterial degeneration ; the 

 work they perform is the chief cause of their rapid decay, for 

 their legs wear out before their bodies : but, apart from this, 

 degenerative changes in the teeth, and more particularly the 

 wearing away of the molars, prevent many horses from reaching 



' Cooper, The Sexual Diaahililiis of Man, &c., London, 1908. 

 '' Smith, loc. cit. 



