884 THE SENSE ORGANS AND SKIN OF THE HORSE 



are fine crystals of linie salts, embedded in a mucoid substance, and termed otoconia. 

 (2) The cristse acusticae are linear thickenings of the wall of each ampulla of the 

 semicircular ducts. Their structure is similar to that of the maculae. (3)- The 

 spiral organ of Corti (Organon spirale) is an epithelial elevation which is situated 

 upon the inner part of the membrana basilaris, and extends the entire length of the 

 ductus cochlearis. It is very complicated in structure, but consists 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 acoustic meatus. The veins go to the ventral petrosal 

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

 circular ducts, 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 

 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 Common Integument 



The conunon integument (Integumentum commune) is the protective covering 

 of the body, and is continuous at the natural openings with the mucous membranes 

 of the digestive, respiratory, and urogenital tracts. It consists of the skin (Cutis), 

 together with certain appendages or modifications thereof, as hair, horn, feathers, etc. 

 It contains peripheral 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 ex- 

 cretion. 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 (Plic£e cutis) occur in certain situations, and in 

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



The skin is attached to the underlying parts by the subcutaneous tissue or 

 subcutis (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 m. cutaneus or panniculus camosus ; 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 bursas often develop over prominent parts 

 of the skeleton where there is much pressure or friction, e. g., at the olecranon, 

 tuber coxee, tuber calcis. 



Structure. — The skin consists of two distinct strata, viz., a superficial epithelial 

 layer, the epidermis, and a deep connective-tissue layer, the corium. The epi- 

 dermis is a non-vascular, stratified epithelium of varying thickness. It presents 



1 This muscle has been described in the Myology. 



