884 THE SENSE ORGANS AND SKIN OF THE HORSE 
are fine crystals of lime salts, embedded in a mucoid substance, and termed otoconia. 
(2) The cristae acustice are linear thickenings of the wall of each ampulla of the 
semicircular ducts. Their structure is similar to that of the macule. (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 duets, and mediates equilibration. The cochlear nerve givesa 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 common 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, ete. 
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 (Plicxe 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 carnosus; in some regions the 
fibers of the muscle are inserted into the skin, and their contraction twitches the 
skin or produces temporary folds.1. 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 burse often develop over prominent parts 
of the skeleton where there is much pressure or friction, e. g., at the olecranon, 
tuber cox, 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. 
