436 COMPARATIVE PHYSIOLOGY. 



usually till after middle life, when the organism has seen its 

 best days. It seems to indicate, if we judge by the frequency 

 of fatty degeneration after disease, that the protoplasm stops 

 short of its best metabolism, and becomes degraded to a lower 

 rank ; for certainly adipose tissue does not occupy a high 

 place in the histological scale. Such pathological facts throw 

 a good deal of light upon the general nature of fat excre- 

 tion, as it would be better to term it, perhaps, and seem to 

 warrant the view that we have presented of the metabolic pro- 

 cesses. 



Although the nerves governing the secretion of milk have 

 not been traced, there can be no doubt that the nervous system 

 controls this gland also. The iniluence of the emotions on both 

 the quantity and quality of the milk in the human subject and 

 in lower animals is well known. 



Comparative. — While breeders recognize certain foods as 

 tending to fat formation and others to milk production, it is 

 interesting to note that their experience shows that race and 

 individuality, even on the male side, tell. The same conditions 

 being in all respects observed, one breed of cows gives more 

 and better milk than another, and the bull is himself able to 

 transmit this peculiarity ; for, when crossed with inferior breeds, 

 he improves the milking qualities of the latter. Individual 

 differences are also well known. 



THE STUDY OF THE METABOLIC FROCfaSSES BY 

 OTHER METHODS. 



It will be abundantly evident that our attempts to follow 

 the changes which the food undergoes from the time of its 

 introduction into the blood until it is removed in altered form 

 from the body has not been as yet attended with great success. 

 It is possible to establish relations between the ingesta and the 

 egesta, or the income and output which have a certain value. 

 It is important, however, to Temember that, when quantitive 

 estimations have to be made, a small error in the data becomes 

 a large error in the final estimate ; one untrue assumption 

 may vitiate completely all the conclusions. 



In discussing the subject we shall introduce a number of 

 tables, but it will be remembered that the results obtained by 

 one investigator differ from those obtained by another ; and 

 that in all of them there are some deviations from strict ac- 



