OF THE HAIRS. 179 



the medullary cells, whether they are quite closed or not, are of different 

 sizes, the aeriferous medulla appearing sometimes coarsely, sometimes 

 finely granular. I have also seen cases in which the medullary cells 

 obviously contained only a single large air-vesicle, and appeared almost 

 like small fat-cells. Very frequently single larger or smaller spots may be 

 observed in the medulla, which contain no air, and are thence pale, and 

 this is constantly the case in the lowermost part of the medulla, close 

 above the bulb. 



The medulla and the cortex are widely different if we compare the 

 extreme forms of their elements ; in the one case we have rigid homo- 

 geneous elongated plates, almost without contents, in the other rounded 

 vesicles filled with fluid or air. If, however, we take into account all 

 their conditions, we shall find that the limits are not so marked, and in 

 fact are often hardly distinguishable. On the one hand, for instance, 

 the medullary cells are not unfrequently of an elongated or short fusi- 

 form figure, whilst on the other the plates of the cortex present a con- 

 siderable cavity containing pigment. If such plates contain, instead of 

 pigment or the smaller air-vesicles, air in a larger cavity, as occurs 

 sometimes though not frequently, it is still more difficult to distinguish 

 the two kinds of elements from one another, and the more so if, as in red 

 hairs, the medulla and cortex are in places, or for considerable distances, 

 not distinctly defined from one another, the superficial cells of the 

 medulla being scattered and passing quite gradually into the plates of 

 the cortex, which lie very close together and contain much air. It is 

 not intended to imply, by this, that the medulla and the cortex are 

 identical, but only that transitions exist, and that the differences which 

 occur are less marked than is commonly supposed. 



The diameter of the medulla is generally, in proportion to that of the 

 hair itself, as 1 : 3-5 ; relatively and absolutely, it is thickest in short 

 thick hairs, thickest in the down and hairs of the head. In a transverse 

 section it presents a round or flattened figure, and the cells which com- 

 prise it are disposed in 1-5 or even more longitudinal series. 



The medullary substance, the cells in which were first accurately de- 

 scribed by G. H. Meyer, varies most of all the constituents of the hair. 

 In the down of the hairs of the head, it has been stated, by some, that 

 it is wholly absent, which is to be corrected thus far, that it is certainly 

 generally absent in the former, and frequently in the latter, perhaps 

 more frequently, in certain individuals. In white hairs, even those of 

 the head, of a tolerable length and thickness, I have never failed to find 

 it always beautifully distinct. In rare cases the medullary tract is 

 double throughout (Bruns, figure in Hassall), more frequently divided in 

 places into two tracts, which soon unite again. In the lower part of 

 the root, the medulla, which is here clear, is often thicker, and exhibits 



