98 



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 ckitin. 



120. 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 



theUai "ceils°" i" colls bundles of fibers or single fibers between the cells. 



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



salivary glands or pan- , ,, , ., ,, i i- n-i ■ j. 



creas showing zymogen bundles; while ycllow, clastic fibers are,m most cases, 

 granules accumulated single and, since they branch and anastomose, or 

 Tdjlcenfto rhe^kimen! ^un together, tend to form a network. The fibrous 

 or cayity, of the gland, tissues also serve to bind parts together and to hold 

 fheTtSe ifl'lT them in place. Another type of supporting tissue is 

 tebrate showing the cartilage, in which the space between the cells is 

 rdT^'^e"™!""," occupied by a substance Icnown as chondrin, or 

 the lumen of the intes- "gristle." Still 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. 37). 



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 



B 



Fig. 36. — Figures 

 to illustrate the secre- 



