366 EXTERNAL INTEGUMENT. 



form of the cells changes successively, from being polyhedral 

 and nearly of the same dimensions on all sides, and becomes 

 first broader, and finally quite flat, being reduced to a simple 

 band : these successive modifications of form may be toler- 

 ably well represented by a series of parabolic lines placed 

 near each other, running in opposite directions, and crossing 

 each other more or less obliquely according to the level of 

 the layers of cells with which their points of intersection 

 correspond (Fig. 94). 



b. Life of the globular Elements of the Epidermis. Be- 

 side the change of form, an important element of difference 

 between the layers is their change in strueture, in composi- 

 tion : the malpighian layer, and the few layers next to it, are 

 formed of actual globules, that is, albuminous masses of 

 protoplasm, capable of being dissolved into mucus, and are, in 

 short, living globular elements ; above these layers, however, 

 the structure suddenly changes, and we find only dried-up, 

 shrivelled or flattened cells, which have lost the greater part 

 of their albumen : they are, in short, corneous cells (corneous 

 layer), the albumen being oxidized and changed into Jcer- 

 atine. 1 



Beside these differences in structure and composition be- 

 tween the two parts of the epidermis, we find, as we should 

 expect, quite as marked a difference in their physiological 

 functions. The superficial corneous or horny cells may be 

 considered as no longer living: the globules of the deep 

 layers are essentially alive ; that is, they react under the in- 

 fluence of excitants, and actually give rise to inflammatory 

 phenomena: thus, if heavy pressure be long continued, the 

 deep layer is metamorphosed and liquefied, giving out either 

 a simple fluid, containing a few nuclei (blister, phlyctence), 

 or else purulent matter ; cold and extreme heat produce the 

 same effect, as do also some chemical irritants (such as can- 

 tharidine), known under the general name of vesicants or 

 vesicatories ; in this case the middle layer of the epidermis is 

 liquefied, forming a fluid mass which raises the cuticle or 

 corneous layer. If this layer be removed, the serum will 

 flow out, and a white covering be seen spread over the der- 

 mis. This is the malpighian layer; and is ready to form 

 again, by its proliferation, the various layers of the normal 



1 Keratine, which is a substance peculiar to the hair, nails, and 

 hoofs, really forms a separate element, being insoluble in potash, 

 unlike other organic substances (Oh. Robin) . 



