Hi EPITHELIUM. 



According to one view (adopted by Paget, Kolliker, Funke, and others), the pale 

 corpuscles gradually become flattened, acquire coloured contents, lose their nuclei, and 

 shrink somewhat in size, and thus acquire the characters of the red disks. But Mr. 

 Wharton Jones has, from an extended series of observations, arrived at the conclusion 

 that, whilst in birds, reptiles, and fishes, the pale or lymph corpuscle, suffering merely 

 some alteration of form and contents, becomes the red disk, its nucleus alone is 

 developed into the red disk of mammalian blood. According to this view (supported 

 by Busk, Huxley, and Gulliver), while the red corpuscle of oviparous vertebrata is the 

 transformed pale corpuscle its development not proceeding beyond this stage the 

 non-nucleated red disk of man and mammalia is, on the other hand, considered to be, 

 not the homologue of the oval nucleated red disk of the oviparous vertebrata, but 

 that of its nucleus. It is not within the scope of this work to enter upon a discussion 

 of the relative merits of these opinions, and the reader is referred to physiological 

 works for a consideration of these and other views adopted by various authors upon 

 the point at issue. 



EPIDERMIC, EPITHELIAL, OR CUTICULAR TISSUE. 



General nature and situation. It is well known, that when the skin is 

 blistered, a thin, and nearly transparent membrane, named the cuticle or 

 epidermis, is raised from its surface. In like manner, a transparent film 

 may be raised from the lining membrane of the mouth, similar in nature to 

 the epidermis, although it has in this situation received the name of 

 "epithelium;" and under the latter appellation, a coating of the same 

 kind exists on nearly all free surfaces of the body. It is true that in 

 many situations the epithelium cannot be actually raised from the subjacent 

 surface as a coherent membrane, still its existence as a continuous coating 

 can be demonstrated ; and, although in different parts it presents important 

 differences, it has in all cases the same fundamental structure, and its 

 several varieties are connected by certain common characters. 



The existence of a cuticular covering in one form or other, has been 

 demonstrated in the following situations : viz. 1. On the surface of the 

 skin. 2. On mucous membranes ; a class of membranes to be afterwards 

 described, which line those internal cavities and passages of the body that 

 open exteriorly, viz., the alimentary canal, the lachrymal, nasal, tympanic, 

 respiratory, urinary, and genital passages ; as well as the various glandular 

 recesses and ducts of glands, which open into these passages or upon the 

 surface of the skin. 3. On the inner or free surface of serous membranes, 

 which line the walls of closed cavities in the head, chest, abdomen, and 

 other parts. 4. On the membranes termed synovial within the joints. 

 5. On the inner surface of the blood-vessels and lymphatics. 



Structure in general. This tissue has no vessels, and, except in certain 

 parts of the organs of the senses, is devoid of nerves, and of sensibility ; it, 

 nevertheless, possesses a decidedly organised structure. Wherever it may 

 exist, it is formed essentially of nucleated cells united together by cohesive 

 matter, often in too small quantity to be apparent. The cells, in 

 whatever way they may be produced, make their appearance first in the 

 deepest part of the structure, where they receive material for growth from 

 the blood-vessels of the subjacent tissue ; then, usually undergoing con- 

 siderable changes in size, figure, and consistency, they gradually rise to the 

 surface, where, as shown at least in various important examples, they are 

 thrown off and succeeded by others from beneath. In many situations the 

 cells form several layers, in which they may be seen in different stages of 

 progress, from their first appearance to their final desquamation. The 

 layer or layers thus formed, take the shape of the surface to which they are 



