GROUP OF THE CUTANEOUS SECRETIONS 893 



It is to be emphasized, however, that this chemical theory does not ex- 

 clude filtration nor osmosis and diffusion as causative factors, but 

 merely states that these processes are modified by certain intracellular 

 reactions, the nature of which is as yet not fully understood. Heiden- 

 hain included the latter under the term of vitalism. It is to be clearly 

 understood, however, that this term does not refer to metaphysical 

 phenomena, but simply to a still inexplicable vital activity of the 

 substance of the cells. This implies that the latter do not act merely 

 as passive filters, but influence the secretion by their metabolic changes. 



Since the internal secretory organs will be more fully described in 

 a subsequent chapter, the present discussion may be restricted to the 

 ''external" secretions. Several of these have already been alluded to 

 in the preceding paragraphs, for example, the cerebro-spinal fluid, 

 the intraocular fluid, the tears and the fatty material of the Meibomian 

 follicles of the eyelids. There still remain to be considered the sweat, 

 the milk, the mucous secretion of the buccal and oral glands, as well as 

 the lymphatic secretions and the very important group of the digestive 

 juices, formed by the saliva, gastric juice, duodenal juice, bile, pan- 

 creatic juice and intestinal juice. 



The Skin as an Organ of Protection. — The skin consists of the epi- 

 dermis or cuticle and dermis or cutis vera. The former appears as a 

 layer of stratified epithelium measuring 0.08 to 0.12 mm. in thickness. 

 Its deepest layer or rete Malpighii is composed of protoplasmic nucle- 

 ated cells, possessing a cylindrical shape. The cells of the surface 

 layer, on the other hand, are hard and horny, non-nucleated, flat- 

 tened chips which are constantly discharged, their places being taken 

 by new cells arising from the rete Malpighii. While being gradually 

 pushed outward, the latter assume the physical and chemical char- 

 acteristics of the surface cells Pigment cells are found in the deeper 

 cells of the epidermis as well as in those of the corium; iii the former, 

 however, the pigment is disseminated while in the latter, it is restricted 

 to particular cells. From here this pigment is said to migrate into 

 the more external Malpighian layer, a contention which fully explains 

 the fact that the skin of a white person, grafted to a negro, presently 

 becomes thoroughly pigmented. 



With the exception of the palms of the hands, soles of the feet, 

 dorsal surfaces of the last phalanges, glans penis and certain parts of 

 the labia, the skin is beset with larger and smaller hairs. These ap- 

 pendages are epidermal growths contained in the skin pits or hair 

 follicles. The part within the follicles is known as the hair-root. 

 Physically hairs are characterized by their marked elasticity and co- 

 hesion, which properties render them capable of supporting a weight 

 of as much as 60 grams. They are very resistant, because composed 

 of a pigmented,' horny, fibrous material; and are strongly hygroscopic, 

 a property which explains the painful sensations generally experienced 

 in scars during wet weather. The gradual change in the color of the 

 hairs may be caused either by a diminution in the amount of their pig- 



