CHEMIC COMPOSITION OF THE HUMAN BODY. 17 



Cerebrin, CifHgsN.Os, is found largely in the brain, in nerves, and 

 in pus-corpuscles. It is a soft, white, amorphous powder, insoluble 

 in water, but swelling up like starch in boiling water. When boiled 

 with dilute acids, it is decomposed, yielding a fermentable dextro- 

 rotary sugar, identical with galactose. Cerebrin may, therefore, be 

 regarded as a glucosid. 



THE PROTEIDS. 



The proteids constitute a group of organic bodies which are found 

 in both vegetable and animal tissues. Though present in all animal 

 tissues, they are especially abundant in muscles and bones, where 

 they constitute twenty per cent, and thirty per cent, respectively. 

 Though genetically related, and possessing many features in common, 

 the different members of the proteid group are distinguished by 

 characteristic physical and chemic properties. 



The average percentage composition of several proteids is shown 

 in the following analyses : 



C. H. N. O. S. 



Egg-albumin . 52.9 7.2 15.6 23.9 0.4 (Wiirtz). 



Serum-ablumin 53.0 6.8 16.0 22.29 1.77 (Hammersten). 



Casein . . . 53.3 7.07 15.91 22.03 0.82 (Chittenden and Painter). 



Myosin . . . 52.82 7.11 16.77 21.90 1.27 (Chittenden and Cummins). 



The molecular composition of the proteids is not definitely known, 

 and the formulae which have been suggested are therefore only ap- 

 proximative. Leow assigns to albumin the formula C72H 112 N 18 O 22 S, 

 while Schiitzenberger raises the numbers to Ca-ioHsgsNetjO^Sa, either of 

 which shows that the proteid molecule is extremely complex. As a 

 class, the proteids are characterized by the following properties : 



i. Indiffusibility. None of the proteids normally assumes the crys- 

 talline form, and hence they are not capable of diffusing through 

 parchment or an animal membrane. Peptone, a product of the 

 digestion of proteids, is an exception as regards its diffusibility. 

 As met with in the body, all proteids are amorphous, but vary in 

 consistence from the liquid to the solid state. The colloid charac- 

 ter of the proteids permits of their separation and purification from 

 crystalloid diffusible compounds by the process of dialysis. 

 3 



