STRUCTUKE OF THE CUTICLE. 211 



and soda. Fat has been obtained by analysis of pulmonary mucus, reputed 

 healthy. 



The mucus is secreted partly by the g-eneral surface of the membrane, partly 

 by special racemose and tubular glands. It is produced -within the epithelium- 

 cells, and accumulates in them during the inten'als of secretion, in the columnar 

 epithelium principally near the free extremity. On the addition of "u-ater the 

 mucus swells up. and may eventually burst through the end of the cell, appearing 

 on the sru-face as a pellucid drop. The nucleus and the greater part of the cell- 

 protoplasm are usually forced towards the attached extremity-. It is in this 

 manner that the so-called " goblet-cells " appear to be produced : occasionally, in 

 the process of preparing a mucous membrane for microscopical examination, 

 large numbers of the cells may undergo the transformation (fig. 1:59 *). 



Reg-eneration. — The reparatory process is active in the mucous membranes. 

 Breaches of continuity occasioned by sloughing, ulceration, or other causes, 

 readily heal. The steps of the process have been exammed with most care in the 

 healing of ulcers of the large intestine, and in such cases it has been found that 

 the resulting cicatrix becomes covered -R-ith epithelium, but that the tubular 

 glands are not reproduced. 



The mucous membrane of the uterus becomes disintegrated and cast off diuing 

 each menstrual flow, and is completely regenerated before the next monthly 

 period. The process of destruction and renovation has recently been carefully 

 traced and described by Dr. John AVilliams (Proceedings of the Royal Society, 

 April, 1874). 



THE SKIN. 



The skin consists of the cutis vera or coriura, and the cuticle or 

 epidermis. 



EPIDERMIS, CUTICLE, OR SCARF-SKIN. 



This belongs to the class of stratified epithelia, the general nature 

 of which has been already considered. It forms a protective cover- 

 ing over every part of the true skin, and is itself non-vascular. The 

 thickness of the cuticle varies in different parts of the surface, 

 measuring in some not more than -rr^Tjth, and in others from J^th 

 to ^Vth of an inch. It is thickest in the palms of the hands and 

 soles of the feet, where the skin is much exposed to pressure, and 

 it is not improbable that this may serve to stimulate the subjacent 

 true skin to a more active formation of epidermis ; but the diflerence 

 does not depend solely on external causes, for it is well marked even in 

 the foetus. 



Structure. — The cuticle is made up of cells agglutinated together in 

 many irregular layers. The deepest cells are elongated in figure, and 

 placed pei^pendicularly on the surface of the corium (figs. 140 Z* and 144) ; 

 they are denticulate at their lower ends, and fit into corresponding fine 

 deuticulations of the corium into which they appear to send processes 

 (fig. 144). The perpendicular cells generally form one, but in some 

 places two or three strata ; above them are cells of a more rounded 

 sha})e. These, as first shown by Max Schultze and Virchow, are 

 marked on their surface with numerous ridges and furrows (in some 

 cases with minute spines). The ridges of neighbouring cells do not, 

 however, interlock one with another as Avas at first supposed, but on 

 the contrary, meet at their summits so as to leave between the cells 

 fine channels through which it is conceived nutrient plasma may be 

 conducted to the cells. These ridged cells form several strata ; above, 

 they become gradually more flattened conformably to the surface until 

 a laver is reached in which it becomes difficult in sections to trace the 



p 2 



