USE, SOURCE, AND DEPOSIT OF FAT. 245 



OF NUTRITION. 



Interstitial death and retrograde metamorphosis imply removal ; but, 

 besides the removals of wasted material, on account of its in- Nutrition for 

 ability to be any longer subservient to the uses of the econ- J^mon ^^ u ' 

 omy, there are also subordinate removals, which are con- modeling. 

 nected with the necessary remodeling of parts. Thus, during the growth 

 of the skeleton, bone earth is transferred from one point to another, the 

 osseous cavities enlarged or altered, and the substance taken from them 

 is carried to other points where it is needed. Under such circumstances, 

 the disappearing part is not, in reality, giving rise to useless products. 

 The substance thus taken from the position it occupied is as valuable as 

 it ever was, and accordingly it is employed over again. 



The restoration of material in the place of that which is being con- 

 sumed for use, and even the preservation of excesses which may be of 

 value at a future time, is very well illustrated by the deposit of fat in the 

 adipose tissue. Transference from point to point of material which has 

 undergone no deterioration may be studied in the history of the growth 

 and development of bone. To these cases in succession I propose to 

 direct attention. 



First. Of the use, sources, and manner of deposit of the fat. 



The use of fat in the animal economy doubtless depends on its heat- 

 making power ; for, though there are many different varieties Physiological 

 of this substance, solid and liquid, they are all characterized relations of fat. 

 by an analogy of composition, all containing a great excess of unoxidized 

 hydrogen. It is, indeed, on this peculiarity that their employment in 

 domestic economy depends. They are all highly combustible, and evolve 

 so much heat as to be very available for the production of flame. 



For the better understanding of the functions discharged by fatty sub- 

 stances, we may perhaps profitably offer the following statement of their 

 chemical relations. 



When a fat or oil is acted upon by an alkali, in contact with water at 

 its boiling-point, decomposition ensues, a fatty acid and gly- Chemical 

 cerine being disengaged, and the acid, uniting with the alkali, cuiiarities of 

 gives origin to a soap. During this action no oxygen is ab- at ' 

 sorbed, but, since the compounds arising present an increase of weight, it 

 is evident that there has been an assimilation of water. In view of these 

 facts, it is therefore inferred that the oils and fats are composed of a fatty 

 acid united with the oxide of a radical, to which the designation of lipyl 

 has been given, and which, when it is displaced, combining with water, 

 gives origin to glycerine. 



Glycerine, which is a substance of considerable physiological import- 

 ance, is a pale yellow liquid, of a sweetish taste, and attracting moisture 



