ANIMAL OUGAMZATION. 



101 



From this circumstance, and also from the obli- 

 quity of the terminations of the sudorific ducts, 

 which produces the effect of valves, the epidermis 

 is impervious to fluids, although capable of im- 

 bibing moisture hygrometrically, and of slowly 

 transmitting a portion to the subjacent textures. 

 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 become dry and unlit for use : and 

 accordingly a separation of this outward layer ge- 

 nerally takes place from time to time, the loss being 

 speedily repaired by a fresh growth iVom the sur- 

 face in contact with the skin. This process is often 

 performed periodically, as is most remarkably ex- 

 emplified in serpents. 



Special provisions are made for preserving the 

 cuticle in a healthy condition ; and more parti- 

 cularly for defending it from the injurious action 

 of the surrounding element. These sometimes con- 



ption of the minute structure of the skin will 

 be better understood by reference to 

 the annexed representation of a per- 

 pendicular section made through all 

 the layers of the skin: (Fig. 28*), 



where D is the derm or corium ; 

 M, the corpus mucosum, gradually 

 passing into E, the epidermis ; P, 

 the papillse ; B, the blennogenous ; 

 C, the chromatogenous ; and S, the 

 sudorific glands, with their respec- 

 tive ducts -. and L, the lymphatic 

 absorbents, not reaching the sur- 

 face. 



