76 PHYSIOLOGY CHAP. 



however, is superfluous, seeing that the exact opposite is upheld 

 by every physiologist ; the centres, that is certain parts or 

 nuclei of grey matter, are totally inexcitable to direct stimuli, 

 and have in other parts (as the so-called excitable area of the 

 cerebral cortex) and other nuclei of grey matter, like the nerves 

 along their course, a much higher liininal excitability than that of 

 the peripheral endings of the afferent nerves. 



The theory of the central origin of hunger and thirst has thus 

 no advantage over that of its local or peripheral origin, and has no 

 such physiological foundation as would force us to regard it as a 

 necessary complement or integration of the general theory of these 

 sensations. 



One objection that seems serious at first sight might be made 

 to the theory of the local origin of hunger. Patients who have 

 successfully undergone almost total extirpation of the stomach do 

 not lose the capacity for feeling hunger ; in fact they crave for 

 nutriment in the shape of milk or other foods, preferably liquid, 

 more frequently than normal individuals. This objection, how- 

 ever, disappears when it is remembered that in this operation it is 

 always necessary to leave a greater or less portion of the cardiac 

 region, which probably contains the most sensitive part of the 

 gastric mucous membrane ; and that in any case the sensory 

 nerves of the stomach, while normally the most excitable to the 

 stimulus of hunger, are not the only nerves capable of transmitting 

 this impulse to the medulla oblongata. The afferent nerves of 

 the intestinal tract are also capable of the same function, and 

 become active when hunger is intense. Obviously they can 

 convey to the centres the craving for food after an operation of 

 gastrectomy. 



,^__ V. Just as the alimentary wants are teleologically co-ordinated 

 with the preservation of the individual, so sexual desire is corre- 

 lated with the preservation of the species. This desire is felt 

 vaguely and indefinitely from early childhood ; it acquires increas- 

 ingly definite and localised characteristics ; finally it becomes 

 imperative when the genital organs suddenly arrive at maturity, 

 that is at the epoch of puberty. The whole organism then under- 

 goes a crisis ; the genital organs become the starting-point of new 

 sensations, till then unknown, which more or less involve the 

 whole nervous system, and are signalised by a pronounced altera- 

 tion of the intellect, feelings, character, and tastes. 



Both in the male and in the female the commencement of 

 sexual maturity is marked by a complex of organic and physio- 

 logical characteristics in addition to the full development of the 

 genital organs, such as the development of the larynx and change 

 of voice (which becomes deeper, more sonorous, and expressional), 

 the growth of the beard and other hairy appendages, the develop- 

 ment of the breasts, appearance of menstruation, etc. 



