THE STRUCTURE OF THE ELEMENTARY TISSUES. 51 



b. That part of the fat which is situate beneath the skin must, by 

 its want of conducting power, assist in preventing undue waste of the 

 heat of the body by escape from the surface. 



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

 to form a soft and yielding yet elastic material wherewith to wrap ten- 

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

 such structures may lie, not endangered by pressure. As 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 mar- 

 row, 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. 



BASEMENT MEMBRANES. 



Basement membranes are a special structure upon which the epi- 

 thelium of mucous membranes rests. They are of homogeneous appear- 

 ance, and are developed from flattened connective-tissue corpuscles, 

 joined at their edges, or from a concentrated cement substance. Some 

 basement membranes possess elasticity, e.g., in the cornea. 



II. CARTILAGE. 



General Structure of Cartilage. All kinds of cartilage are composed 

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

 ences 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 invested by a 

 thin but tough firm fibrous membrane called the pericliondrium. 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. 



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 

 human body investing the articular ends of bones, and forming 

 the costal cartilages, the nasal cartilages, and those of the larynx with the 

 exception of the epiglottis and cornicula laryngis, as well as those of 

 the trachea and bronchi. 



Structure. Like other cartilages it is composed of cells imbedded in 



