622 



ANATOMY OF VERTEBRATES. 



sively absorbed, and the base of the quill is contracted in dia- 

 meter, until it adheres to the surface of the derm by a narrow 

 neck, below which is a slightly expanded remnant of the matrix. 

 The disposition of the dermal muscles subserving the spiny armour 

 of Erinaceus europ&us, is given at pp. 18, 19, figs. 7 and 8. 



In the Porcupine (Hystrix cristata) the spines attain so great a 

 length that they are called ' quills.' The formative pulp, fig. 489, 

 e, is longitudinally furrowed ; to it is due the cellular pith : the 

 capsule or inner layer of the theca is reflected into, or fills, the 

 pulp-grooves, and deposits therein, and continuously around the 

 whole, the horny cortex : the consequent arrangement of crust 

 and pith is such as in transverse section to give the figure, fig. 489. 

 Beneath the matrix is a cavity like a minute ' bursa mucosa,' which 

 allows much freedom of motion to the quill when acted upon by 

 the muscle, d, of the sheath, /: a sebaceous gland, A, serves the 

 quill opening into the sheath near the outlet. When the growth 

 is completed, the matrix shrinks, and the same movement to the 

 periphery of the derm takes place as in the spines of the Hedge- 

 hog. Thus it happens that when the quills of the Porcupine are 

 violently shaken by the action of the cutaneous muscle, c, the 

 adhesion of some old quills to the derm has been so reduced that 

 they are thrown off. 



489 



Section of skin, with matrix and root of quill : i, section of quill, Porcupine. 



362. Scales. Only one genus of Mammal (Manis) offers a 

 covering of scales ; and with them are associated hairs. The scales 

 are large, epidermal or horny in tissue, and imbricate or overlap- 

 ping, with the free border turned backward, vol. ii. fig. 158. The 

 external surface of the derm is raised into large rhomboidal pro- 

 cesses, upon which the scales are moulded : beneath the derm is 

 a thick 'panniculus carnosus,' adapted to draw the integument 

 around the animal as a means of defence, and connected with 

 muscular slips, which erect the scales. 



