364 HISTOLOGY 



middle region that contains the pigment, and an inner or principal region 

 that seems to serve as the chief means of mechanical support. The 

 strata of the inner region are hardest in the outer portion, and become 

 softer as they are examined nearer the cells. The innermost is but 

 weakly calcined. 



In general, the cuticle, in its office of mechanical protection, is often 

 found to bear modified portions, as lumps, ridges, knobs, hairs, spines, 

 etc., of complicated patterns and varying sizes. The simplest form 

 of this modification is where a cuticle-like structure that ordinarily ex- 

 tends in an unbroken sheet over the surface is more highly developed 

 in certain locations, forming isolated areas, rods, cones, spikes, or one 

 or another of an almost infinite variety of structures scattered more or 

 less regularly over the surface of the integument. In all these cases the 

 structures in question are formed like other cuticle by the action of the 

 epidermal cells. Sometimes one, sometimes many, of these cells form 

 each such a structure, and frequently the structure acquires a secondary 

 use. The strange cuticle-like structure found in the gizzard of the bird, 

 the stomach-jaw of the Arthropoda, and the other organs of mastication 

 of a cuticular nature have been treated of under the tissues of mastica- 

 tion (Chapter XV). 



Turning our attention again to protection against abrasion by an 

 epithelium, we find that stratification (Chapter VI) is, in its essential fea- 

 tures, a highly developed means of mechanical protection. The struc- 

 tural devices that a stratified epithelium develops to accomplish its ends 

 are much the same as in the simple epithelium. They are simply modi- 

 fied to meet the different structure of this form of epidermis. 



A cuticle is not formed as an extracellular structure in the stratified 

 forms of epithelia. But the outer cells of the epithelium are modified 

 so greatly that they form layers of different structure and consistency 

 that are admirably adapted to all purposes that a cuticle could fulfill. 

 Only in this case it is the cells themselves that perform the work, and not 

 an extracellular material that they have formed. The similarity of the 

 two processes is much heightened by the fact that the cells that do this 

 duty are dead themselves and might be compared very closely with a 

 cuticle from the standpoint of function, far removed as they are from it 

 in their origin. This form of outer protection is also somewhat more 

 convenient than the cuticle of a simple epithelium because it does not 

 have to be shed at intervals, except in a very few cases, but is continually 

 and gradually dropped and renewed. For a very simple sort of strati- 

 fication see Chapter VI, where this is illustrated in a chaetognath worm 

 Sagitta. An example of this principle, carried to a large degree of effi- 

 ciency without any great specialization, is furnished in the skin of man. 

 This is pictured in Figure 330. A small region of this epidermis is here 



