32 



constant regular exercise our muscles enlarge, become thicker, 

 heavier, contain more solid matter, in other words they have 

 gained in nitrogen, and this is doubtless to provide for future 

 similar demands upon their strength, 



Pettenkofer and Voit have also shown that during rest a 

 certain amount of storage of oxygen goes on, especially during 

 sleep, and this storage no doubt occurs chiefly in the muscles 

 themselves, and the supply is available at the time of exercise. 



The evidence of a storage of nerve power is less distinct than 

 in the case of the muscles. Still it is rendered probable by the 

 phenomena attending the discharge of refuse material from the 

 body and the function of parturition. It is possible also, by means 

 of a galvanic current, to exhaust the nerve power of a muscle or 

 even of a nerve of sensation, and a considerable time elapses before 

 it is restored, showing that a certain accumulation of this force must 

 take place during rest. The brain, again, is certainly a storehouse 

 of nervous energy. We talk of the stores of knowledge and of 

 erudition possessed by certain gifted persons, and although we are 

 ignorant of the precise nature of memory, we may regard it as a 

 case in point. Whether memory depends upon actual " residua " 

 impressed upon the nervous elements, or whether it consists in the 

 more ready conveyance of impressions that have once or more 

 times travelled along a certain course, in either case we may 

 properly regard it as a sort of storage of force, or, at least, as the 

 facilitation of its manifestation, when required at future times. 

 The discharge of phosphorus in the secretion from the kidneys, after 

 severe mental or bodily exertion, is a further proof of the using up 

 of previously stored up material in the nervous system. 



But perhaps the most important forms of the storage of 

 material are to be found, as in plants, in the provisions made for 

 the growing germ or for the preservation of the embryo in animals. 

 The egg of an oviparous animal is only a type of the ovum of a 

 viviparous one. 



There is indeed some difference in the mode of segmentation 

 of the yelk, and in birds the contents of the yelk-sac afford nourish- 

 ment until the end of incubation. In mammalia its office ceases at 

 an early period, owing to the close connection soon formed between 

 the embryo and its mother. But in both the function of the yelk is 

 the same ; it is a store of nutriment provided until the time arrives 

 at which other sources of food are opened up. 



In all the tissues of the young animal also provision is made 

 for future growth. 



But this is probably not nearly all that is involved in the pro- 

 cess of reproduction. 



That there are some still more subtle processes of storage 

 involved in the work of reproduction is evident from the facts 

 respecting inheritance — atavism, prepotency, reversion, alternation 

 of generation, and so on. 



