798 THE APPARATUS OF THE SENSES. 



In the Sheep, real hair, not wool, is found on the lower part of the face, 

 and the extremities of the limbs. 



In the Goat, the hairs of the beard are very long, and compose a dis- 

 tinctive tuft; this animal has also a fine crisp duvet or down beneath the 

 ordinary hair. 



In the Pig, the bristles are very strong in the region of the back : in old 

 animals they are usually bi- or trifurcated at their free extremity ; there 

 also exists a fine soft hair on this animal. It has no tentacular hairs. 



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.") 



STRUCTURE. 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, That of the hair- 

 follicle. 



1. The hair presents a free portion, the shaft, and another concealed in 

 the follicle, the root ; the latter widens at its base the bulb of the hair to 

 embrace the papilla or hair-germ. 



Three superposed layers compose a hair. The epidermis is a thin 

 lamella of horny 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 forms the largest part of the thickness of the hair. 

 It is striped longitudinally, and provided with pigment granules, whose 

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

 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 microscope. 

 Treated by potass or sulphuric acid, the cortical substance is reduced to 

 elongated spindles, which again may be decomposed into epithelial lamellae 

 narrow, and with nuclei. On arriving at the root, the cells change their 

 character, becoming polyhedric, filled with fluid, and exhibiting a perfectly 

 distinct nucleus and more or less pigment. The medullary substance 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 granulations 

 and air globules. 



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, externally, 

 a loose conjunctival layer, analogous to the reticular layer of the derma ; 

 next, an internal derniic layer, dense and close like the papillary layer of 

 the skin ; an amorphous limiting membrane ; an epidermic zone, the external 

 sheath of the hair, formed by cells, similar to those of the rete mucosum ; and 

 a second epidermic zone, the internal 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 hair-germ, is a small, conical, vascular, and nervous 



