100 



METAZOA IN GENERAL 



other cases the secretion involves the destruction of the whole cell, which 

 pours out its contents to form the secretion; examples are milk glands 

 and sebaceous, or oil, glands. 



When epithelial cells undergo a change which makes them hard, the 

 substance formed is horn, which chemically is a substance called keratin. 

 In this fashion true horns, claws, nails, and tortoise shell are developed. 

 In the case of teeth and some scales of vertebrates, however, enamel may 

 be the substance produced. In some cases epithelia produce a hard 

 covering by the hardening of a secretion; an example of such a hard 



covering is the cuticula of the bodies of insects, which 

 contains chitin. 



122. Supporting and Connective Tissues. — These 

 tissues are found in all parts of the body and differ 

 from other tissues in the fact that the character of 

 the tissue depends not so much upon the cells which 

 it contains as upon nonliving intercellular materials 

 formed by secretion from these cells. Examples of 

 such materials are fibers, bone, and cartilage. Most 

 of the embryonic connective tissue appears in the 

 form of a network of branched cells and is known as 

 mesenchyme. 



A prominent function of these tissues is support, 

 either of the body as a whole or of some particular 

 part. Among supporting tissues having this func- 

 tion are fibrous tissues, which are characterized by 

 JheUa^ceTii?" A^ cdis bundles of fibers or single fibers between the cells, 

 of such a gland as the White, nonelastic fibers are usually collected in 

 ifeZLt^'lZZn bundles, while yellow, elastic fibers are, in most cases, 

 granules accumulated single and, sincc they branch and run together, tend 

 idiltnfto ttmt to form a network. The fibrous tissues also serve 

 or cavity, of the gland, to bind parts together and to hold them in place. 

 tt'TntStilTe ;f;'r- Another type of supporting tissue is cartUage, in 

 tebrate showing the which the space between the cells is occupied by a 

 accumulation of mucus g^bg^ance known as chondrin, or "gristle." Still 



and its extrusion into _ _ _ ' ^ 



the lumen of the intes- another type is bone, in which there is laid down 

 *"^®' between the cells a deposit of salts of lime which 



makes the tissue very firm and capable of giving effective support to 

 even a large body. Special types of fibrous tissue which also serve to 

 bind parts together include the ligaments, which connect the parts other 

 than muscles, and the tendons, which serve to connect muscles to other 

 parts at their point of attachment (Fig. 40). 



An additional function which these tissues have, and which also is a 

 passive function, is to store fat. Fat tissue is simply a connective tissue 

 in which the cells, because they are filled with great globules of fat, have 



Fig. 39. — Figures 

 to illustrate the secre- 



