THE SKIN 761 



essentially of remarkable supporting cells and hair cells. Fibers of the cochlear 

 nerve ramify about the basal parts of the hair cells. 



Vessels and Nerves.— The artery of the internal ear is the internal auditory 

 artery, a very small vessel which usually arises from the posterior cerebellar artery, 

 and enters the internal auditory meatus. The veins go to the inferior petrosal 

 sinus. The vestibular nerve is distributed to the utricle, saccule, and membranous 

 semicircular canals, and mediates equilibration. The cochlear nerve gives a branch 

 to the saccule and enters the central canal of the modiolus. Along its course it 

 gives off fibers which radiate outward between the two plates of the lamina spiralis 

 ossea, and ramify about the hair cells of the organ of Corti. The ganglion spirale 

 or ganglion of Corti is situated in the spiral canal of the modiolus near the fixed 

 border of the lamina spiralis. The cochlear nerve mediates the sense of hearing. 



The Skin 



The skin or common integument (Integumentum commune) is the protective 

 covering of the l)ody, and is continuous at the natural openings with the mucous 

 membranes of the digestive, respiratory, and urogenital tracts. It contains per- 

 ipheral ramifications of the sensory nerves, and is thus an important sense organ. 

 It is the principal factor in the regulation of the temperature of the body, and by 

 means of its glands it plays an important part in secretion and excretion. Some 

 of its special horny modifications or appendages are used as organs of prehension or 

 as weapons. 



The thickness of the skin varies in the different species, on different parts of 

 the body of the same animal, and also with the breed, sex, and age. The color 

 also varies greatly, but this is masked in most places by the covering of hair or wool. 

 The skin is in general highly elastic and resistant. 



Permanent folds of the skin (Plicae cutis) occur in certain situations, and in 

 some places there are cutaneous pouches or diverticula (Sinus cutis). 



The skin is attached to the underh'ing parts l^y the subcutis or superficial 

 fascia (Tela subcutanea). This consists of connective tissue containing elastic 

 fibers and fat. When the fat forms a layer of considerable thickness, it is termed 

 the panniculus adiposus. Over a considerable part of the body the subcutis con- 

 tains striped muscle, the panniculus camosus (M. cutaneus); in some regions the 

 fibers of the muscle are inserted into the skin, and their contraction twitches the 

 skin or produces temporary folds. ^ The amount of subcutaneous tissue varies 

 widely; in some places it is abundant, so that the skin can be raised considerably; 

 in other situations it is practically absent and the skin is closely adherent to the 

 subjacent structures. Subcutaneous bursae often develop over prominent parts 

 of the skeleton where there is much pressure or friction. 



Structure. — The integument consists of the cutis or skin proper and its epider- 

 mal appendages, e. g., hairs, hoofs, claw^s, horns, etc. The cutis consists of tw^o 

 distinct strata, viz., a superficial epithelial layer, the epidermis, and a deep connec- 

 tive-tissue layer, the coritrai. 



The epidermis is a non-vascular, stratified epithelium of varying thickness. It 

 presents the openings of the cutaneous glands and the hair-follicles, and its deep 

 surface is adapted to the corium.^ It is divisible into a superficial, harder, drier 

 part, the stratvun comeum, and a deeper, softer, moister part, the stratimi germin- 

 ativimi. The cells of the latter contain pigment, and by their proliferation com- 

 pensate the loss by desquamation of the superficial part of the stratum comeum. 



^ This muscle has been described in the Myology. 



2 To prevent a possible misapprehension, it may be stated that the epidermis primarily 

 molds the corium, and that the glands and hair-follicles are invaginations of the epidermis. 



