1032 VEETEBKATED ANIMALS 



appear dark by transmitted and white by reflected light. The cells 

 of the medullary axis in particular are very commonly found to 

 contain air, giving it the black appearance shown at C. The 

 difference between the blackness of pigment and that of air-spaces 

 may be readily determined by attending to the characters of the 

 latter as already laid down, and by watching the effects of the 

 penetration of oil of turpentine or other liquids, which do not alter 

 the appearance of pigment spots, but obliterate all the markings 

 produced by air-spaces, these returning again as the hair dries. In 

 mounting hairs as microscopic preparations they should in the first 

 instance be cleansed of all their fatty matter by maceration in 

 ether, and they may then be put up either in weak spirit or in 

 Canada balsam, as may be thought preferable, the former menstruum 

 being well adapted to display the characters of the finer and more 

 transparent hairs, while the latter allow the light to penetrate more 

 readily through the coarser and more opaque. Transverse sections 

 of hairs are best made by glueing or gumming several together and 

 then putting them into the microtome ; those of human hair may 

 be easily obtained, however, by shaving a second time, very closely, 

 a part of the surface over which the razor has already passed more 

 lightly, and by picking out from the lather, and carefully washing, 

 the sections thus taken off. 1 



The stems of feathers exhibit the same kind of structure as hairs, 

 their cortical portion being the horny sheath that envelopes the 

 shaft, and their medullary portion being the pith -like substance 

 which that sheath includes. In small feathers this may usually be 

 made very plain by mounting them in Canada balsam ; in large 

 feathers, however, the texture is sometimes so altered by the drying 

 up of the pith (the cells of which are always found to be occupied 

 by air alone) that the cellular structure cannot be demonstrated 

 save by boiling thin slices in a dilute solution of potass, and not 

 always even then. In small feathers, especially such as have a 

 downy character, the cellular structure is very distinctly seen in the 

 lateral barbs, which are sometimes found to be composed of single 

 files of pear-shaped cells, laid end to end ; but in larger feathers 

 it is usually necessary to increase the transparence of the barbs, 

 especially when these are thick and but little pervious to light, 

 either by soaking them in turpentine, mounting them in Canada 

 balsam, or boiling them in a weak solution of potass. In feathers 

 which are destined to strike the air with great force in the act of 

 flight, we find each barb fringed on either side with slender flattened 

 filaments or ' barbules ; ' the barbules of the distal side of each barb 

 are furnished 011 their attached half with curved hooks, whilst those 

 of the proximal side have thick turned-up edges in their median 

 portion ; as the two sets of barbules that spring from two adjacent 

 barbs cross each other at an angle, and as each hooked barbule of 

 one locks into the thickened edge of several barbules of the other, 

 the barbs are connected very firmly, in a mode very similar to that 



1 On the minute structure of hair, consult Grimm's Atlas der menscliliclieii nnd 

 tierischen Haare (Lahr, 1884, 4to, with a preface by W. Waldeyer). 



