66 ANIMAL PHYSIOLOGY. 



epithelium. When very thin sections vertical to the surface 

 are made through it, and examined under the microscope, 

 it is seen to consist of two parts, which are very different in 

 appearance : a deep part consisting of delicate texture, and a 

 superficial which is horny. The difference may be made 

 very striking by acting on the specimen with a drop of an 

 ammonia solution of carmine, which takes 110 effect on the 

 horny part, but stains the deep part, and particularly the 

 nuclei, it being the property of that solution so to act on 

 all growing nucleated corpuscles. 



The deep part of the cuticle is likewise called the 

 mucous layer (rete mucosum, or Malpighian layer), and 

 it presents several strata of cells. Those which lie 

 deepest, resting on the cutis vera, are always somewhat 

 elongated vertically, while those immediately superficial 

 to these are small, and the remaining strata exhibit 

 cells larger and more flattened the nearer they are to 

 the surface. It appears, therefore, that whatever may be 

 the mode of origin of the deepest or elongated cells, the 

 other layers consist of elements, the history of each of which 

 is, that it has originated as one of the minutest cells, and 

 passes gradually to the surface as it enlarges, undergoing 

 both change of shape and chemical composition, until it 

 becomes incorporated with the horny part of the cuticle, and 

 is ultimately shed from the surface in the shape of a small 

 scale. 



The superficial or horny part of the cuticle consists of 

 flattened cells closely adherent one to another, receiving 

 additions from the mucous layer on the deep side, and cast- 

 ing off its oldest cells from the suiface in a perpetual in- 

 sensible desquamation. Its cells may be separated and their 

 nuclei displayed by the action of a solution of caustic potash. 

 The explanation of blistering is, that the mucous layer, acted 

 on by some unwonted irritation, pours out a serous discharge, 

 which, being pent up by the impermeable horny part of the 

 cuticle, separates it from its connections. 



The cuticle, besides the mechanical protection which it 

 gives the body, furnishes likewise, by the impermeability of 

 its horny layer, and the intervention of living parts between 

 t^p surface ajad the vascular channels within, a protection 



