108 THE MECHANICAL FUNCTIONS. 



Sometimes, instead of being cylindrical, the fila- 

 ments are more or less flattened, striated, deeply 

 grooved, or even beaded. Instead of being solid, 

 they may even be tnbular : and they exhibit also 

 the greatest diversity in their length, fineness, tena- 

 city, rigidity, and disposition to curl. All these va- 

 rieties may be traced to corresponding differences in 

 the form, and the relative 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 that of 

 merely covering the body, and which are even sup- 

 plied with nerves, converting them into instruments 

 of a sense of touch. 



In the quills of the Porcupine a still more com- 

 plicated organization has been detected. Fig. 33 

 shows a quill with its bulbous root, detached from 

 the body ; and Fig. 34, a transverse section magni- 

 fied. The bulb itself is contained in a distinct cell, 

 shown at (a), Fig. 35, which represents a longi- 

 tudinal section of these organs. This cell contains 

 a portion of fat in which the numerous vessels sup- 

 plying 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 



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

 Porcupine, in the Nouvelles Annales du Museum, i. 429. 



