91 



Hassall has figured a hair with a double medullary cavity. This is an 

 evident monstrosity, and one which I presume to be very rare, as I have 

 not yet met with a single instance. 



When the hair has attained its highest development, we find it, 

 apparently, a much more complex structure than we have hitherto 

 described. In that of the peccaii, e. g. which is so large as to have the 

 appearance of bristles, we see a regular system of stays passing from 

 one side to the other, and evidently intended to give strength and 

 support to the out«r coating. 



This is better seen in the section of the hair of the porcupine and hedge- 

 hog, where the structures are so dense as to have received the name of 

 quills, and so lai'ge as to be recognised by the naked eye. But this ap- 

 parent complexity soon disappears when we consider the central part as 

 nothing more than an excessive cell development, with an increase of 

 density of the cell walls in particular directions. 



Having spoken now of the soft hair of the deer, and the large hair of 

 the peccari, and also of the hard and dense spine of the porcupine and 

 hedgehog, let us examine into the essential difference between hair 

 and bristles, and why it is, that one large hair is soft and another hard. 



As we might anticipate, the difference is not so much, in actual struc- 

 ture, as in the relative growth and thickness of the cortical or fibrous, 

 and the cellular parts. In those we have as yet considered, the latter 

 has been developed to the greatest extent. In bristles and all hard hairs, 

 the outer coat increases without any great increase of cellularity in the 

 interior. There is a dense exterior covering, and only a single small 

 row of central cells, which have been gradually decreasing since their first 

 appearance. This will readily be understood by the following table : — 



Ratio of Cortical and Cellular Structure, in Diameters. 



OOTER. iknrb. 



Hare s Fur, small hairs I to 1^- — 2 



„ „ large hairs 1 „ 6-7 



Musk Deer, large hairs 1 „ 60 



Red Deer, „ 1 „ 70 



Porcupine 1 „ 910 



Human Hair 10 „ 1 



Eyelash of Whale 17 „ 1 



Pig's Bristle about 100 „ 1 



Horse Hair about 100 „ 1 



Not only, however, are the bristles of the hog tribe strengthened by an 

 enormous development of their outer coat; their resistance is still further 



