120 THE MECHANICAL FUNCTIONS. 



live actions of the component parts of the bulb, 

 namely, the pulp and its capsule.* 



The structure of the organs by which hairs 

 are formed is not easily distinguished, in the 

 ordinary kinds of hair, on account of their 

 minuteness : it is readily seen, however, in the 

 large whiskers of the feline species, and also of 

 the seal, which are subservient to more extended 

 uses than those of merely covering the body, 

 and which are even supplied with nerves, con- 

 verting them into instruments of a sense of 

 touch. 



In the quills of the porcupine a still more 

 complicated organization has been detected. 

 Fig. 33 shows a quill with its bulbous root, de- 

 tached from the body ; and Fig. 34, a transverse 

 section magnified. The bulb itself is contained 

 in a distinct cell, shown at A, Fig. 35, which 

 represents a longitudinal section of these organs. 

 This cell contains a portion of fat in which the 

 numerous vessels supplying its pulp and capsule 

 are embedded. The bulb is itself surrounded 

 by an outer sheath, s, into the cavity of which, 

 B, there opens a duct, D, proceeding from a small 

 cell or follicle, F, lodged in the cellular substance 

 on the outside of the sheath. This upper cell 

 communicates below with another cavity, c, 



* See F. Cuvier's Memoir on the Formation of the Quills 

 of the Porcupine, in the Nouvelles Annales du Museum, I. 429, 



