enter.  No other living organism is so resistant to changes in environment as is man, and to this resistance he owes his supremacy.  By means of his intelligence he can change the environment.  He is able to resist the action of cold by means of houses, fire and clothing; without such power of intelligent creation of the immediate environment the climatic area in which man could live would be very narrow.  Just as disease can be acquired by an unfavorable environment, man can so adjust his environment to an injury that harmony will result in spite of the injury.  The environment which is necessary to compensate for an injury may become very narrow.  For an individual with a badly working heart more and more restriction of the free life is necessary, until finally the only environment in which life is even tolerably harmonious is between blankets and within the walls of a room.

The various conditions which may act on an organism producing the changes which are necessary for disease are manifold.  Lack of resistance to injury, incapacity for adaptation, whether it be due to a congenital defect or to an acquired condition, is not in itself a disease, but the disease is produced by the action on such an individual of external conditions which may be nothing more than those to which the individuals of the species are constantly subject and which produce no harm.

[Illustration:  Fig. 3.—­A section of the skin. 1.  A hair.  Notice there is a deep depression of the surface to form a small bulb from which the hair grows. 2.  The superficial or horny layer of the skin; the cells here are joined to form a dense, smooth, compact layer impervious to moisture. 3.  The lower layer of cells.  In this layer new cells are continually being formed to supply those which as thin scales are cast off from the surface. 4.  Section of a small vein. 9.  Section of an artery. 8.  Section of a lymphatic.  The magnification is too low to show the smaller blood vessels. 5.  One of the glands alongside of the hair which furnishes an oily secretion. 6.  A sweat gland. 7.  The fat of the skin.  Notice that hair, hair glands and sweat glands are continuous with the surface and represent a downward extension of this.  All the tissue below 2 and 3 is the corium from which leather is made.]

[Illustration:  Fig. 4.—­Diagrammatic section of A surface showing the relation of glands to the surface. (a) Simple or tubular gland, (b) compound or racemose gland.]