MUCUS. 511 



raw silk. It can be obtained pure most readily by re 

 peatedly digesting silk with water at 30, and treating 

 the bright-yellow, lustrous residue with alcohol^ and 

 ether. By boiling with dilute sulphuric acid, it yields 

 tyrosin, leucine, and some glycocol. 



In addition to fibroin, silk contains a species of gela 

 tin, in many respects similar to glutin, silk-gelatin 

 (seracin), C 15 H 25 N 5 8 , which can be extracted by boiling 

 water. It is formed apparently from fibroin by the 

 assimilation of oxygen and water. In a dried condi 

 tion, it forms a colorless and inodorous powder, which 

 swells up largely with water, and dissolves in it more 

 readily than glutin. A solution which contains less 

 than 1 per cent, still congeals on cooling, forming a 

 consistent jelly. By long boiling with dilute sulphu 

 ric acid, it yields a little &quot;leucine, about 5 per cent, of 

 tyrosin, and about 10 per cent, of serine (p. 175). 



11. Fat. 



Fat occurs in a great many forms in the animal 

 organism, partially as minute drops or globules, sus 

 pended in fluids, as in the milk, in blood, partially de 

 posited in a free state in the tissues or inclosed in par 

 ticular fat cells ; in the latter manner for instance in 

 the upper portion of the subcutaneous cellular tissue. 



In connection with glycerin it has already been men 

 tioned, that the fats which are most widely distri 

 buted in the animal kingdom are identical with the 

 vegetable fats of most general occurrence. In the 

 same connection, the details in regard to the occurrence 

 of the various animal fats, their properties and com 

 position, were given. 



12. Mucus. 



In the mucus secreted by mucous membranes are 

 detected microscopical clear granules, and separated 

 cells or particles of the external coat (epithelium) of 

 the mucous membranes. 



The characterizing ingredient of mucus is a peculiar 



