HAIRS, FEATHERS, AND SCALES 19 



obliquely across it, coming out beyond the middle, where 

 it thins away to an edge. The outline is not circular, but 

 elliptical; that is, the hair is not round, but flattened. 

 There is no separable cortex, or bark, and the whole sub- 

 stance appears made up of excessively fine fibres, of which 

 we see the ends cut across. A rough dark line occupies 

 the middle of the slice, in the plane of the greater diameter; 

 but at the edge of the slice we are able to see that this is 

 not a solid core, as has been sometimes supposed, but a 

 cavity passing up through the hair. It is surrounded by 

 a layer of medullary cells, which appear black, because they 

 are filled with air. 



The finer hairs of the Horse and the Ass, such as those 

 selected from the cheeks, have the sinuous edges of the 

 plates about as close as in human hair. But they are dis- 

 tinguished at once by the conspicuousness of the medullary 

 portion, which is thick, and quite opaque, and is broken 

 up (especially toward each extremity of the hair) into sepa- 

 rate longitudinal irregular masses. 



The fine wool of the Sheep is clothed with imbrications, 

 proportionally much fewer than those of human hair, while 

 the diameter is also much less. Thus these examples, se- 

 lected from fine flannel and from coarse worsted, vary 

 in diameter from s^th to ^^th of an inch; and there are, 

 upon an average, about two imbrications in a space equal 

 to the diameter. No color is perceptible in these speci- 

 mens ; they are as transparent and colorless as glass. The 

 imbricated plates project here considerably more than in 

 either of the examples we before examined; the "teeth," 

 however, form an obtuse angle. 



We shall presently see the importance of this imbri- 

 cate structure; but we will first look at a few more ex- 



