CHAMER XVII. 



OP SECBETION AND' EXCRETION IN GENERAL. 



EvHRY ofganised and livifig element assimilates and dissimilates 

 incessantly. But in proportion as tte organism is complicated 

 and differentiated, the conditions of assimilation become more 

 difficult ; and this function needs for its accomplishment the 

 auxiliary apparatus, digestion, circulatory and respiratory, which 

 we have passed in review. Besides, assimilation would be im- 

 possible if the waste of the immediate principles were not ezpulsed 

 in the degree of its production. In short, dissimilation is the 

 indispensable corollary of assimilation, and like this last it needs 

 special apparatus wherever the structure of the organism is too 

 complex. These special apparatus are the glands of which we 

 have by and by to give a general description. Nevertheless along 

 with the special organs of expulsion, the elementary property of 

 simple exhalation still continues. The most marked example of 

 this primitive mode of elimination is certainly the gaseous and 

 aqueous exhalation, which is so extensively effected by means of 

 the lung. 



But besides the glands serving for simple expulsion, there are 

 others whose distinctive office consists in forming, at the expense 

 of the nutritive liquids, special substances, destined to play a 

 part in the accomplishment of one of the grand functions of the 

 liviag organism. The glands of the first type are called glands 

 of excretion, the others, glands of secretion. We may give as an 

 example of the first the glands which excrete urine and sweat, 



