INTKKMKIMATK PRODUCTS. 73 



3. Gelatin is produced by boiling fibrous connective tissues, 

 such as ligaments, tendons, the true skin and bones in water. On 

 cooling, the fluid forms a jelly, which can be dried to a colorless 

 brittle body which swells in cold water and dissolves on being 

 heated. It is not precipitated by acetic acid, but yields precipi- 

 tates with chloride of mercury or tannin, which latter is seen in 

 making leather. On boiling with sulphuric acid it yields glycin 

 and leucin but no tyrosin. 



4. Elastin is obtained from yellow elastic tissue by boiling with 

 caustic alkalies. It is little affected by boiling water, strong acetic 

 acid, or weak alkalies, but dissolves in concentrated sulphuric 

 acid. It is precipitated by tannin, and yields leuciu when boiled 

 with sulphuric acid. 



5. Keratin exists in the epidermic appendages (hair, horn, nails, 

 etc.). It is like the albuminous bodies in containing a consider- 

 able quantity of sulphur, but differs from them and the other albu- 

 minoids in general properties. It is soluble in alkalies, swells 

 in strong acetic acid, gives the xanthoproteic reaction, and is 

 extremely insoluble in the digestive juices. 



Group IV. PRODUCTS OF TISSUE CHANGE. 

 Intermediate or bye products. 



These are, no doubt, protoplasmic manufactures destined for 

 some useful purpose, but they do not long exist in their original 

 form, being often broken up into other compounds they are reab- 

 sorbed, or pass away with the faeces. These bodies are found in 

 the various secretions. Most of them, however, can be better 

 described with the function of the gland which forms the secre- 

 tion in which they occur. 



Attention must be here drawn, however, to certain complex 

 bodies existing in the bile. Some complex nitrogenous substances 

 and the monatomic alcohol, cholesterin, will also be now men- 

 tioned. But the reader must remember that chemically they are 

 not connected with the other bodies the description of which 

 immediately follows theirs, namely, the effete products. 



Bile Salts. Two acids exist in the bile united with soda to form 

 soluble soap-like salts. They may be recognized by the purple- 



