DIGESTION. 267 



nitrogenous waste from the body, more recent researches tend 

 to show that this is not the case. The nitrogenous waste is 

 chiefly excreted in the form of urea in the urine and sweat. 



Connective Tissue. This material, which constitutes the 

 main ingredient in the tendons, ligaments, cartilages, skin, 

 &c., of the animal body, consists essentially of gelatine-yield- 

 ing substances, of which the following are the chief : 



(1) E las tin, an insoluble substance containing little or no 



sulphur. Its composition, according to analyses quoted 

 by Hammarsten, is about 



Carbon. Hydrogen. Nitrogen. Oxygen. 



54-3 7-2 16-7 21-8% 



(2) Collagen, an insoluble body yielding gelatine by long 



boiling with water. It contains about '6% sulphur. 



Hammarsten gives the following : 



Carbon. Hydrogen. Nitrogen. 



Collagen ... 50-75 6-47 17-86 24-92% 

 Gelatine ... 50-00 6-50 17-50 26-00% 



On decomposition it yields albuminous substances and 

 a large quantity of glycocoll [amido-acetic acid, CH 2 

 (NH 2 ).COOHJ , sometimes called sugar of glue. 



(3) Keratin. This is the chief constituent of horns, hoofs, 



skin, feathers, hair, wool, nails, &c. Obtained from 

 different sources it shows a variation in composition ; 

 in general it resembles collagen or gelatine, but differs 

 from those substances in containing much more sul- 

 phur (4 or 5%), part of which is in a very loose state of 

 combination and may be removed by alkalies or even 

 by boiling with water. Keratin is insoluble in w r ater, 

 alcohol, or ether, but can be dissolved by heating with 

 water to 150 or 200 under pressure. 



DIGESTION. The food of an animal is rarely in a form 

 capable of direct absorption into the system. Before it can be 

 taken into the blood stream and utilised in the body it is 

 usually necessary that certain chemical changes should be 

 produced so as to render its constituents soluble and diffusible. 



