438 CHEMICAL DUALITIES OF THE CUTICLE. 



of cuticle has received an entirely new aspect, from recent 

 observations, and especially from those of Henle.* He has 

 shown that with very few exceptions, all the free surfaces of 

 the body not only the skin which has its cuticular covering 

 but those of the serous cavities, the mucous passages,! the 

 blood-vessels, and the ducts of the glands, are invested by a 

 membrane, composed of one, or more layers of primary cells, 

 forming a delicate cuticle or epithelium. The epidermis, 

 cuticle, or external covering of the skin, when examined with 

 a powerful microscope, is seen to be composed of several layers 

 of cells, which 'are the consequence of an uninterrupted process 

 of exudation which has place upon the corion or true skin. 

 This exudation, though unorganized, retains some vital proper- 

 ties, and is a cytablastema ; that is, a basis structure, or soil/ 

 from which new growths or developments take place. These 

 new growths are cytoblasts, or cell germs ; that is, cells or 

 vesicles, at first globular, afterwards lenticular and opaque, 

 (each one surrounding a central nucleus), which possess within 

 themselves the inherent principles of growth.^ The more 

 recently produced cells, which of course are those in contact 

 with the corion, are like all young cells, spherical in their 

 figure ; they become flattened as they develop themselves 

 and approach the surface ; so that when examined on a sec- 

 tion, they are found to have undergone changes of form from 

 that of a globular cell, provided with a nucleus, to that of 

 a flat scale, in which no trace of a nucleus appears, and which 

 lay, one over another, like so many layers of tiles or pavement. 

 The innermost layers are soft and cellular; the outer ones 

 become dried on the surface from exposure to the air, and fall 

 off in squamae or scales. 



* Allgera. Anat., p. 260. 



f Vide Gen. Anat. of Serous and Mucous Membr. Vol. 2. 



f The cell germs, here and in other parts of the body, bear a general rela- 

 tion to the size of the blood globules of the same individual. This is a remarka- 

 ble fact, and somewhat in favor of the views of Dr. Barry, who states that it is 

 the blood disks that are transformed into these cell germs. Phil. Trans. Part 

 11. 1840. 



Henle, makes three varieties of epithelium or cuticle, 1st. The pavement- 

 ed sqnamous or tesselated epithelium, above described, found on the skin, serous 



