CARTILAGE. 43 



deep layer of the epidermis, or the so-called rete mucosum. (See 

 chapter on the Skin.) 



The pigmentary matter is a very insoluble compound of 

 carbon, hydrogen, nitrogen, and oxygen, the carbon largely 

 predominating ; besides, there is a small quantity of saline 

 matter. 



The uses of pigment in most parts of the body are not clear. 

 In the eyeball it is evidently intended for the absorption of 

 superfluous rays of light. 



Cartilage. 



Cartilage or gristle exists in different forms in the human 

 body, and has been classified under two chief heads, namely, 

 temporary and permanent cartilage ; the former term being ap- 

 plied to that kind of cartilage which, in the foetus and in 

 young subjects, is destined to be converted into bone. The 

 varieties of permanent cartilage have been arranged in three 

 classes, namely, the cellular, the hyaline, and the fibrous carti- 

 lages, the last-named, being again capable of subdivision into 

 two kinds, namely, elastic or yellow cartilage, and the so-called 

 fibro-cartilage. 



Elastic cartilage, however, contains fibres, and fibro-carti- 

 lage is more or less elastic ; it will be well, therefore, for dis- 

 tinction's sake to term those two kinds white fibro-cartilage 

 and yellow fibro-cartilage respectively. 



The accompanying table represents the classification of the 

 varieties of cartilage : 



1. Temporary. 



f A Cellular. 



r White fibro-cartilage. 

 C. Fibrous. fibro . cartila ge. 



All kinds of cartilage are composed of cells imbedded in a 

 substance called the matrix : and the apparent differences of 

 structure met with in the various kinds of cartilage are more 

 due to differences in the character of the matrix than of the 

 cells. Among the latter, however, there is also considerable 

 diversity of form and size. 



With the exception of the articular variety, cartilage is in- 

 vested by a thin but tough and firm fibrous membrane called 

 the perichrondrium. On the surface of the articular cartilage 

 of the foetus, the perichondrium is represented by a film of 

 epithelium ; but this is gradually worn away up to the margin 

 of the articular surfaces, when by use the parts begin to suffer 

 friction. 



