



THE INTEGUMENTS AND CLOTHING OF MAMMALIA. 151 



being formed. The greyness of hair, from age, results from a deficiency 

 both of nutriment and colouring matter : the loss of hair, in old age (often, 

 indeed, at an earlier period), not followed by a renewal, is in consequence 

 of the secreting energies of the arterial system having begun to fail. 



Bristles and spines are modifications of hair: they assume various 

 forms, and are more or less rigid. In a genus among the Rodentia 

 (Echimys), the spines, which are mixed with soft hairs, are bayonet- 

 shaped (fig. 150, of which a is a section) : in the Hedgehog (fig. 151), 

 and the Porcupine (fig. 152, of which 6 is an enlarged section), these 

 spines are sharp, rigid, and highly developed. 



150 151 



152 



Bristles of Echimys, Hedgehog, and Porcupine. 



If the skin of the Hedgehog be examined, the quills, with their roots 

 and sockets, may be seen extending to different depths, according to the 

 period of their growth. The newly-formed ones are deep, and terminate 

 in a broad basis ; but, as the growth of the quill proceeds, the reflected 

 integument, forming the socket, contracts, and draws the quill nearer the 

 surface. The pulp is, at the same time, progressively absorbed, so that, 

 at last, the quill is attached to the surface of the skin by a narrow neck 

 only, with a small terminal head, for firmer adhesion, below which the 

 remains of the socket and sheath exist in the form of a small bulb. 



A similar mode of growth is exemplified in the spiny quills of the 

 Porcupine : these spines consist of a smooth, glossy envelope of horn, and 

 an inner pith, or medullary substance, of a soft texture, and of a pure 

 white. They grow from a bulbous root, formed within a cell below the 

 cutis, and containing, also, a portion of fat, in which the vessels, supplying 

 its pulp and capsule, are imbedded. The capsule consists of two mem- 

 branes, of which the innermost secretes the horny envelope, while the pulp 

 supplies the pith of the spine. The bulb is, itself, surrounded with a 

 membranous sheath, into the cavity of which, as discovered by F. Cuvier, 

 a duct opens, proceeding from a follicle, seated in the adjacent cellular 

 tissue, and partially divided into an upper and lower cavity, filled with 

 an unctous matter, which, during the formation of the spine, is transmitted 

 through the duct, either as a lubricating fluid, or as entering into its 

 composition. 



The diagram (fig. 153), representing the bulb of the quill, or spine, 

 of the Porcupine, may serve to illustrate the foregoing remarks : , 

 is a cut section of the root, in a capsule, 6, consisting of two mem- 



