STRUCTURE OF THE COPJUM. 213 



detached, appearing then like threads proceeding from its under 

 surface. "When a portion of the cuticle is destroyed by a blister, or 

 otherwise, it is readily regenerated. 



Chemical composition.— The cuticle consists principally of a substance 

 peculiar to the epithelial and homy tissues, and named hcrat'in. This homy 

 matter is insoluble in water at ordinary temperatiu-es, and also in alcohol. 

 It is soluble in the caustic alkalies. In comi^osition, it is analogous to the 

 albuminoid principles, but wdth a somewhat larger i^roportion of oxgyen ; like 

 these, it contains sulphur. Besides keratin, the epidermis yields, on analysis, a 

 small amount of fat, ^Wtli salts, and traces of the oxides of iron and manganese. 

 The tissue of the cuticle readily imbibes water, by which it is rendered soft, thick, 

 and opaque, l)ut it speedily dries again, and recovers its usiial characters. 



Development and growth, of Epidermis. — In the earliest condition of the 

 embryo there is a special layer of cells, derived from the primitive embryonic 

 cells, set aside for the production of the epidermis ; and it is quite conceivable, 

 and by some histologists considered most probable, that the subsequent generations 

 of eiudermic and epithelial cells by which the tissue is throughout life maintained, 

 are derived by unbroken descent from the original embryonic stratum. At the 

 same time, the reproduction of epidermis in cicatrices after wide and deep de- 

 struction of the subjacent skin, implies some other source of new cells ; unless 

 indeed it be supposed that the new cuticle grows exclusively from the old at the 

 circumference of the sore. Setting aside this supposition, we might conceive the new 

 cells to come from the connective tissue corpuscles, or at least, from migratory cells 

 of the granulating sui-face of the new-growing skin ; and such a mode of reproduc- 

 tion of epidermis has been described as actually observed ; moreover, it may be 

 questioned whether, in certain situations, this may not be the regular process by 

 which the growth of epithelium is maintained : the readiness, howev^, with, 

 which a previously obstinate ulcer will often become covered with epidermis if 

 one or two small portions of that tissue are transplanted to its surface from some 

 other i^art, would indicate, on the other hand, that the presence of some 

 epithelial cells at least is necessary to set U2> the grcn-th. 



WTien the lowermost cells are elongated and vertical, it is difficult to conceive 

 that they rise up as such, and take their place in the upper strata ; for the cells 

 nest aljove them are spheroidal in shape and may be smaller in size. It seems 

 more likely that they divide into or produce the smaller cells. It might be 

 supposed that an oblong vertical cell, by division of its nucleus and separation of 

 the upper portion of the cell-body, produces a new and smaller cell, which rises 

 uj:), while the parent cell maintains its place, and lengthens out again for a 

 repetition of the process. 



COKIUM. 



The true skin, cutis vera, derma, or corium, is a sentient and 

 vascular texture. It is covered and defended, as already explained by 

 the non-vascular cuticle, and is attached to the parts beneath by a 

 layer of areolar tissue, named " subcutaneous," which, excepting in a 

 few parts, contains fat, and has therefore been called also the " panni- 

 culus adiposus " (fig. 155, d). The connection is in many parts loose 

 and movable, in others close and firm, as on the palmar surface of the 

 hand and the sole of the foot, where the skin is fixed to the subjacent 

 fiiscia by nimierous stout fibrous bands, the space between being filled 

 with a firm padding of fat. In some regions of the body the skin is 

 moved by striated muscular fibres, which may be unconnected to fixed 

 parts, as in the case of the orbicular muscle of the mouth, or may be 

 attached beneath to bones or fascia3, like the other cutaneous muscles 

 of the face and neck, and the shori palmar muscle of the hand. 



Structure. — The corium is made up of an exceedingly strong and 



