THE HAIR. 389 



vessel walls. These vasomotor nerve-fibers are neuraxes of sym- 

 pathetic neurones. 



In aquatic birds, and more especially in ducks, the waxy skin of 

 the beak and the cornified portion of the tongue contain the so- 

 called corpuscles of Herbst, which resemble the Pacinian corpuscles 

 in general structure, but have cubical cells in the core. In the same 

 tissues are also found the corpuscles of Grandry, 60 fj. long and 

 40 IJL broad. They consist of a thin connective-tissue capsule, con- 

 taining two or three large cells. The nerve-fiber retains its medul- 

 lary sheath for some distance within the capsule. The axis-cylinder 

 ends in discs situated between the cells inclosed by the capsule. 



B. THE HAIR. 



The hair and nails are regarded as special differentiations of the 

 skin. Hair is found distributed over almost the entire extent of the 

 skin, varying, however, in quantity and arrangement in different 

 regions. None whatever is present in the palm of the hand and 

 sole of the foot. In the third fetal month small papillary elevations 

 of the skin are seen to develop in those areas in which the hairy 

 growth later appears. Under each of these elevations there occurs 

 a proliferation of the cells of the Malpighian layer downward into 

 the corium. Although the elevations soon disappear, the epithelial 

 ingrowth continues and finally forms the hair germ. This is soon 

 surrounded by a connective-tissue sheath from the corium, in which 

 two layers may be distinguished. At the lower end of the hair 

 germ the corium is pushed upward, forming a papilla which pene- 

 trates into the thickened bulb of the germ. This is called the hair 

 papilla. In the mean time the hair germ itself is undergoing marked 

 differentiation. An axial portion, forming later the hair and inner 

 root-sheath, and a peripheral, constituting later the outer root- 

 sheath, are developed. From the latter are derived also the first 

 traces of the sebaceous glands, which in the adult state are in close 

 relationship to the hair and empty their secretion into the space 

 between the hair and its sheath. As soon as the various layers of 

 the hair are complete it grows outward, breaking through the over- 

 lying layers of the epidermis. 



The visible portion of the hair is called the hair shaft, and 

 that portion below the skin is the hair root. The lower portion 

 of the hair resting upon the papilla is known as the hair bulb, 

 and the sheaths encircling the root and bulb are called the root- 

 sheaths, the entire structure constituting the hair follicle. 



The adult hair is covered by a thin cuticle, consisting of over- 

 lying plate-like cells, i.i ft thick, most of which possess no nuclei. 

 Beneath the cuticle is the cortical layer, composed of several strata 

 of long, flattened cells from 4. 5 (i to 1 1 ft broad and provided with 

 nuclei. These are also known as the cortical fibers of the hair. 

 Upon treatment with ammonia the fibers separate into delicate 



