40 HANDBOOK OF PHYSIOLOGY. 



c. As a packing material, fat serves very admirably to fill tip spaces, 

 to form a soft and yielding yet elastic material wherewith to wrap tender 

 and delicate structures, or form a bed with like qualities on which such 

 structures may lie, not endangered by pressure. 



As good examples of situations in which fat serves such purposes may 

 be mentioned the palms of the hands and soles of the feet, and the 

 orbits. 



d. In the long bones, fatty tissue, in the form known as yellow 

 marrow, fills the medullary canal, and supports the small blood-vessels 

 which are distributed from it to the inner part of the substance of the 

 bone. 



II. CARTILAGE. 



Structure of Cartilage. All kinds of cartilage are composed of 

 cells imbedded in a substance called the matrix : and the apparent differ- 

 ences of structure met with in the various kinds of cartilage are more due 



jii^^mijjm 



M^gMiai^jim 



FIG. 45. FIG. 46. 



FIG. 45. Ordinary hyaline cartilage from trachea of a child. The cartilage cells are inclosed 

 singly or in pairs in a capsule of hyaline substance, x 150 diams. (Kleia and Noble Smith.) 

 FIG. 46. Fresh cartilage from the Triton. (A. Rollett.) 



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 invested by a 

 thin but tough firm fibrous membrane called the perichondrium. On 

 the surface of the articular cartilage of the foatus, 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. 



Nerves are probably not supplied to any variety of cartilage. 



Cartilage exists in three different forms in the human body, viz., 1, 

 Hyaline cartilage, 2, Yellow elastic cartilage, and 3, White fibro-cartilage. 



1. Hyaline Cartilage. 



Distribution. This variety of cartilage is met with largely in the 



