152 



INTRODUCTION. 



Capsule and bulb of a cnticiilar spine 

 of a Porcupine 



branes ; the pulp, c, which secretes the spine, has its surface fluted, 

 153 so that the horny matter deposited on it is 



" '-.T moulded into diverging lamina}, carried up the 

 shaft, as the latter is pushed onward, whence 

 the quill has the appearance of being grooved, 

 as, indeed, it would be, were not these la- 

 minae covered with a horny secretion from the 

 membrane of the capsule, as the quill recedes 

 from the pulp : c?, is the cell in which the bulb 

 is contained, and in which the vessels to sup- 

 ply the pulp and the capsule are distribut- 

 ed ; e, is the semi-divided crypt, containing 

 unctuous matter poured into the capsule during 

 the formation of the quill ; f, the cellular tissue ; 

 g, the skin. 



If one of these spines be divided (as b, fig. 152), lines of the corti- 

 cal, glossy matter (black or white, according to the black or white part of 

 the spine which is cut) will be seen converging regularly from the external 

 coat, through the pith, to a central point, to which they are directed, or, vice 

 versa, from which they radiate to the envelope. The colouring matter is 

 only deposited on the glossy or horny envelope, and such is the case 

 with all hairs. It has been supposed, that the colour of the hair depends 

 upon that of the mucous tissue, through which it passes : the truth of 

 this may be doubted. The same cause, whatever that be, which colours 

 the mucous layer black, may also colour the hair, or bristles. In 

 variegated individuals of the Hog, for example, we find the patches of 

 black bristles growing from a black surface, the white bristles from a 

 white surface ; but there is no proof in this, that, 

 because the mucous layer is black, therefore the 

 hair is black. The beautiful blue colour of the 

 skin of the chest and abdomen in several Monkeys 

 has been already alluded to ; the hair of these 

 parts is white : again, in the human subject, jet 

 black hair sometimes accompanies the fairest 

 white complexion. Many animals have the 

 hair ringed with alternate bands of different 

 colours ; the skin beneath being white or whitish ; 

 and, in several, the hairs have a metallic lustre 

 (Aulocodus and Chrysochloris, for example), 

 without any correspondence in the colour of the 



Bristles of Hog. HIUCOUS tisSUC. 



The annexed figure (154) represents a section of the skin on the back of 

 the Hog. The cut surface exhibits the canals in the corium, containing the 



