894 



THE EXTERNAL SECRETIONS 



ment or by the presence of minute air-bubbles within their medulla 

 and fibrous layer. These bubbles reflect the light very strongly. 

 The former change is the usual cause of the grayness of the hair in 

 old age, whereas its sudden turning gray is due principally to the 

 formation of these vacuoles. 



Physiologically, it is of interest to note that the adult human individual pro- 

 duces about 0.20 gram of hair-substance in the course of a day, but this amount 

 may be greatly increased by heat, massage, and the cutting of the hair. Attention 

 has already been called to the fact that numerous smooth muscle cells lie in relation 

 with the hairs which bridge over the angle formed by the obliquely placed roots 

 of the latter and the surface of the skin. The contraction of these muscle fibers, 



therefore, must lead to an erection of the 

 hairs and the peculiar reflex phenomenon, 

 known as "goose flesh." This reaction 

 most commonly arises in consequence of 

 local or general stimuli, such as cold, emo- 

 tions, and irritations within the domain of 

 the autonomic nervous system. A few 

 cases, however, have been recorded which 

 show that the pilo-motor mechanism may 

 be brought under the control of volition. ^ 

 While the question of whether cats and 

 other animals are able to erect their hairs 

 at will, cannot be decided definitely, it 

 seems that this reaction is not always 

 wholly reflex but embraces a strong ele- 

 ment of volition. For the present, how- 

 ever, it must be placed in the group of the 

 perception or association reflexes. The 

 sensitiveness of the hairs which plays so 

 important a part in the sensations of 

 touch, is subserved by a ring-like plexus of 

 nerve fibrils surrounding the hair-follicle. 

 In the second place, it should be noted 

 that the contracton of these muscle cells 

 exerts a certain pressure upon the neigh- 

 boring sebaceous glands, causing them to 

 discharge their oily secretion in greater 

 quantities than before. Moreover, since 

 the ducts of these glands most commonly 

 empty directly into the hair-follicles, a means is provided to keep the hairs soft 

 and pliable. In the third place, it should be remembered that these scattered 

 muscle cells play an important part in determining the vascularity of the skin, 

 because their contraction hinders the passive expansion of the capillaries, thereby 

 keeping the blood in the deeper parts of the body, while their relaxation allows 

 the superficial capillaries to become injected with blood drawn from other organs 

 and tissues. Without doubt, these vascular changes possess an important bearing 

 upon the regulation of the body-temperature, because the relaxation of the cuta- 

 neous capillaries invariably leads to a greater dissipation of heat, and vice versa. 

 In this connection brief mention should also be made of the fact that the effect 

 of cold and warm baths upon the circulation is made possible in part through thesa 

 muscle cells, and in part through the muscle cells of the arterioles themselves. 



The Skin as an Organ of Secretion. — The sebaceous glands are 

 simple acinous in character and are usually found in close relation with 

 1 Maxwell, Amer. Jour, of Physiol., vii, 1902, 369. 



Fig. 487. — Nerve Terminals Around 

 THE Root of the Hair Follicle. 

 M, Medulla; P, papilla; ^.external 

 root sheath; /, internal root sheath; N, 

 ramification of the nerve fiber. 



