1 14 THE MECHANICAL FUNCTIONS. 



nourished by vessels, its outer layer is liable to 

 wear away, or to become, by drying, unfit for 

 use : and accordingly a separation of this out- 

 ward 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 remarkably 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 some- 

 times 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 manner, and continually poured 

 out on the surface, through ducts, the orifices of 

 which are easily seen with the naked eye, dis- 

 posed in a line on each side of the body. 



Connected with the skin, and more particu- 

 larly with the cuticle, are structures of very va- 

 rious forms, intended for giving additional pro- 

 tection, occasionally contributing their aid in 



