402 THE SUPPORTING TISSUES. 



stance, after removal of the mucin, consists of the collagen-fibrils, 

 a few cellular elements, and quite commonly also contains isolated 

 fibrils of the yellow or elastic variety, which can be readily recog- 

 nized on microscopical examination by their higher power of refrac- 

 tion. When placed in water, or, still better, in a dilute solution of 

 acetic acid or caustic alkali, the white fibres swell, while solutions 

 of some of the metallic salts, such as ferric sulphate and mercuric 

 chloride, cause them to shrink. Tannic acid acts in a similar 

 manner. Owing to the great stability of the compound of the latter 

 with collagen, tannic acid is extensively utilized in the preparation 

 of leather. 



On boiling white fibrous tissue in water the collagen dissolves, 

 with the formation of gelatin, which latter separates as a jelly-like 

 mass on cooling. 



Yellow or Elastic Tissue. In the yellow elastic fibres, elastin 

 takes the place of the collagen of the white fibrous variety. For 

 purposes of study, the substance is most conveniently obtained from 

 the ligamentum nucha? of the ox, in which such fibres are almost 

 exclusively found. 



Reticulated Tissue. In the reticulated tissue, which constitutes 

 the fibrous framework of the lymph-glands of the body, but which 

 is also found in the alveoli of the lungs, in the liver, the kidneys, 

 and the intestinal mucous membrane, the fibres consist of reticulin. 



Reticulin is said to have the composition C, 52.88 ; H, 6.97 ; N, 

 15.63; S, 1.88; P, 0.34. It is insoluble in water, alcohol, ether, 

 dilute mineral acids, lime-water, and solutions of sodium carbonate. 

 It resists the action of pepsin and trypsin, and is dissolved only in 

 cold sodium hydrate solution on standing for several weeks. It 

 does not give Millon's reaction, and accordingly yields no tyrosin on 

 hydrolytic decomposition. On prolonged boiling with water or 

 dilute alkalies, its phosphorus is split off; the residual material is 

 then soluble in water, and can be precipitated from its solutions by 

 means of acetic acid. 



CARTILAGE. 



Histologically considered, cartilage consists of a more or less 

 hyalin matrix, in which a variable number of cartilage-cells are found 

 imbeddedc In certain localities, further, a differentiation of the mat- 

 rix into fibres, both of the white and the yellow elastic variety, is 

 observed. Such fibres, as in the case of the corresponding connec- 

 tive tissue, consist of collagen and elastin, respectively. 



Of the composition of the cells nothing specific is known. Appar- 

 ently they contain a small amount of glycogen, which disappears 

 during starvation. Traces of fat are also found. During embryonic 

 life they are quite numerous, but later they diminish in number, and 

 in the adult animal the matrix largely predominates. Embryonic 

 cartilage does not yield gelatin on boiling with water, and it is quite 

 likely that as in the case of the matrix of embryonic connective 



