86 



in their classes of animals, we now come to describe the hairs as met 

 with in the class mammalia. 



Three structures may be distinguished in the great majority of hail's : 

 a cortical or scaly, a horny or fibrous, and a medullary or cellular portion. 

 Each deserves particular description. We will commence with the 

 general characters of the hair, as regards shape and colour. 



The shape of the hair varies considerably, as regards both its long and 

 cross sections, in the same and in different animals. Its usual form is 

 that of an elongated cone, but in some instances there are alternate 

 contractions and dilatations, the terminal one being largest. A transverse 

 section shows that hairs are round, triangular, or square, with the angles 

 rounded. In curly hair, the shape is always, as far as I have seen, kidney- 

 shaped or semiterete ; and we may reasonably account for the curl, either 

 by assuming that there is excess of development on one side, or arrest 

 of growth on the other. In the fur of the hare (Lepus timidus), the 

 shape varies with its size, from a perfect circle to a complete dumbell. 

 The largest hairs commonly assume the latter form, but I have seen one 

 or two circular. 



The colour of hair varies in different animals, sometimes in the same. 

 In the Ichneumon we have hairs marked by broad, dark-brown lines, 

 while the rest of the structure is brownish-yellow. In others, one hair 

 is white, another black, another yellow, as in the Ermine. In ourselves, 

 we have black hairs on our head, and red ones elsewhere, and brown 

 intermixed with white. If we examine, microscopically, upon what this 

 colour depends, we find that in one variety a colouring matter is uni- 

 formly diffused throughout the whole tissue ; in others it is deposited in 

 cells or granules. The colouring matter is usually soluble in liquor 

 potasscB and appears to be of an oleaginous character. In other cases it 

 seems to consist of pure carbon. These coloured spots are by many 

 supposed to be the remains of the original cell, but there is strong 

 reason to believe that they are secreted to a great extent by the hair 

 itself. We shall see that all the primitive cells are colourless. One 

 point in connection with this is well worthy of notice, viz. that it is now 

 well known that hairs have changed their colour, from brown or black 

 to grey, with great rapidity, — 



" The hair has turned wliite 

 In a single night." 



" Deadly fear will time outgo, 

 And blanch at once the hair." 



This can only have been effected by an absorbing or changing power of 

 the hairs themselves ; and, where there is a power of absorption, there is 



