ANIMAL ORGANIZATION-. [)l 



rently homogeneous in its texture and composition. It is 

 impervious to fluids, althougli capable of imbibinsr moisture, 

 and of slowly transmitting a portion to the subjacent tex- 

 tures. Its thickness varies exceedingly in different parts; 

 being adapted to the kind of protection it has to afford 

 against pressure, friction or other causes of injury. As it 

 is not nourished by vessels, its outer layer is liable to wear 

 away, or to become, by drying, unfit for use: and accord- 

 ingly a separation of this outward layer generally takes place 

 from time to time, the loss being speedily repaired by a 

 fresh growth from the surface in contact with the skin. 

 This process is often performed periodically, as is most re- 

 markably exemplified in serpents. 



Special provisions are made for preserving the cuticle in 

 a healthy condition; and more particularly for defending it 

 from the injurious action of the surrounding element. These 

 sometimes consist of a supply of oily fluid, prepared in 

 small cavities that are situated in the skin itself, and have 

 minute ducts opening upon the surface. These cavities, 

 termed sebaceous follicles, are generally interspersed in 

 great numbers on different parts of the body, abounding 

 more especially in those places where folds occur, and where 

 there is the greatest friction. In fishes, mollusca, and other 

 aquatic animals, the skin is at all times defended from the 

 action of the water, by a viscid or glutinous secretion pre- 

 pared in this m.anner, and continually poured out on the sur- 

 face, through ducts, the orifices of which arc easily seen with 

 the naked eye, disposed in a line on each side of the body. 

 Connected v/ith the skin, and more particularly with the 

 cuticle, are structures of very various forms, intended for 

 giving additional protection, occasionally contributing their 

 aid in progressive motion, and sometimes fashioned into 

 weapons of offence. In this class should bo included all the 

 varieties of hair, such as wool, fur, feathers, bristles, quills, 

 and spines, as well as the more ordinary kinds of hair. All 

 these resemble the cuticle in their chemical composition, 

 differing only in their degrees of hardness and condensation. 

 Horn is formed of the same material as hair: as arc also the 



