906 THE SENSORY APPARATUSES. 



In the Dog, the length, fineness, and consistency of the hair depends on the 

 breed. 



In the Cat, the hair in some breeds — as in the Angora— is remarkable for 

 its length and softness. This creature has the tentacula enormously developed 

 as a moustache. 



In "none of these animals is there a " foot-lock.") 



Steucture. — The hairs are implanted in the texture of the derma, and 

 sometimes even in the subjacent tissues, their base being enclosed in a follicle, 

 at the bottom of which their elements are developed. It is therefore necessary 

 to study : 1. The structure of the hair. 2. Thaf. of the hair-folHcle. 



1. The hair presents a free portion — the shaft (scapus), and another con- 

 cealed in the follicle — the root (radix pili) ; the latter widens at its base — the 

 hulh {bulbus pili) of the hair — to embrace the papilla or hair-germ. 



There are three superposed layers in a hair. The epidermis (cuticle) is a thin 

 lamella of homy flattened cells, imbricated like tiles on a roof. Its elements 

 are marked on the surface of the hair by shaded lines anastomosing to form 

 a network ; they enlarge, and become more apparent under the influence of an 

 alkali. The epidermis belongs to the shaft and a portion of the root ; near the 

 bulb it is replaced by soft nucleated cells, which are implanted vertically. 



The cortical substance (cortex) forms the largest part of the thickness of the 

 hair. It is striped longitudinally, and provided with pigment granules, the 

 number of which varies with the colour of the coat. In white hairs these 

 granules are absent, but there are found in them, as well as in coloured hairs, 

 small spaces containing air, and which exhibit a dark colour under the micro- 

 scope. Treated by potassium or sulphuric acid, the cortical substance is reduced to 

 elongated spindles, which again may be decomposed into slender epithelial 

 lamellte without nuclei. On arriving at the root, the cells change their cha- 

 racter, becoming polyhedric, filled with fluid, and exhibit a perfectly distinct 

 nucleus and more or less pigment. The meduUary substance (mechdla) occupies 

 a narrow irregular cavity in the centre of the hair, extending from the bulb 

 or termination of the root, to the point. It has for its base rectangular, rarely 

 circular, cells, which, according to Kolliker, contain fat-granules and air- 

 globules. (Many hairs show no medulla, and even in thick hairs it does not 

 always extend the entire length of the hair.) 



2. The hair-follicle is a narrow cavity, slightly contracted at its orifice and 

 dilated at the bottom, where the hair-papilla is placed. It is a simple involution 

 of the skin, as its structure demonstrates. It presents, from without to within : 



1. A loose conjunctival layer, analogous to the reticular layer of the derma. 



2. An internal dermic layer, dense and close hke the papillary layer of the skin. 



3. An amorphous limiting membrane. 4. An epidermic zone — the external sheath 

 of the hair — formed by cells, similar to those of the stratum mucosum. 5. A 

 second epidermic zone — the interned sheath of the hair — which repeats the horny 

 layer of the epidermis, and is confounded with the termination of the epidermis 

 of the hair towards the lower third of the follicle. 



The papilla, or hcdr-germ, is a small, conical, vascular, and nervous prolonga- 

 tion rising up into the hair-bulb. It furnishes the hair with nutrition and the 

 elements of growth. 



The walls of the follicles of the enormous hairs — or tentacula — which garnish 

 the lips of the Horse, or bristle from those of the Cat (moustaches), are pro- 

 vided with nerve-ramifications which endow these appendages with a high 



