Part 1 (2/2)

[Ill.u.s.tration: 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.]

All of the causes of disease act on the body from without, and it is important to understand the relations which the body of a highly developed organism such as man has with the world external to him.

This relation is effected by means of the various surfaces of the body. On the outside is the skin [Fig. 3], which surface is many times increased by the existence of glands and such appendages to the skin as the hair and nails. A gland, however complicated its structure, is nothing more than an extension of the surface into the tissue beneath [Fig. 4]. In the course of embryonic development all glands are formed by an ingrowth of the surface. The cells which line the gland surface undergo a differentiation in structure which enables them to perform certain definite functions, to take up substances from the same source of supply and transform them. The largest gland on the external surface of the body is the mammary gland [Fig. 5] in which milk is produced; there are two million small, tubular glands, the sweat glands, which produce a watery fluid which serves the purpose of cooling the body by evaporation; there are glands at the openings of the hairs which produce a fatty secretion which lubricates the hair and prevents drying, and many others.

[Ill.u.s.tration: FIG. 5.--A SECTION OF THE MAMMARY GLAND. (_a_) The ducts of the gland, by which the milk secreted by the cells which line all the small openings, is conveyed to the nipple. All these openings are continuous with the surface of the skin. On each side of the large ducts is a vein filled with blood corpuscles.]

[Ill.u.s.tration: FIG. 6.--PHOTOGRAPH OF A SECTION OF THE LUNG OF A MOUSE.

_x x_ are the air tubes or bronchi which communicate with all of the small s.p.a.ces. On the walls of the part.i.tions there is a close network of blood vessels which are separated from the air in the s.p.a.ces by a thin membrane.]

The external surface pa.s.ses into the interior of the body forming two surfaces, one of which, the intestinal ca.n.a.l, communicates in two places, at the mouth and a.n.u.s, with the external surface; and the other, the genito-urinary surface, which communicates with the external surface at one place only. The surface of the intestinal ca.n.a.l is much greater in extent than the surface on the exterior, and finds enormous extensions in the lungs and in the great glands such as the liver and pancreas, which communicate with it by means of their ducts. The extent of surface within the lungs is estimated at ninety-eight square yards, which is due to the extensive infoldings of the surface [Fig 6], just as a large surface of thin cloth can, by folding, be compressed into a small s.p.a.ce. The intestinal ca.n.a.l from the mouth to the a.n.u.s is thirty feet long, the circ.u.mference varies greatly, but an average circ.u.mference of three inches may safely be a.s.sumed, which would give between seven and eight square feet of surface, this being many times multiplied by adding the surfaces of the glands which are connected with it. A diagram of the microscopic structure of the intestinal wall shows how little appreciation of the extent of surface the examination with the naked eye gives [Fig. 7].

By means of the intestinal ca.n.a.l food or substances necessary to provide the energy which the living tissue transforms are introduced.

This food is liquefied and so altered by the action of the various fluids formed in the glands of the intestine and poured out on the surface, that it can pa.s.s into the interior of the body and become available for the living cells. Various food residues representing either excess of material or material incapable of digestion remain in the intestine, and after undergoing various changes, putrefactive in character, pa.s.s from the a.n.u.s as feces.

[Ill.u.s.tration: FIG. 7.--A SECTION OF THE SMALL INTESTINE TO SHOW THE LARGE EXTENT OF SURFACE. (_a_) Internal surface. The small finger-like projections are the villi, and between these are small depressions forming tubular glands.]

By means of the lungs, which represent a part of the surface, the oxygen of the air, which is indispensable for the life of the cells, is taken into the body and carbonic acid removed. The interchange of gases is effected by the blood, which, enclosed in innumerable, small, thin-walled tubes, almost covers the surface, and comes in contact with the air within the lungs, taking from it oxygen and giving to it carbonic acid.

The genito-urinary surface is the smallest of the surfaces. In the male (Fig. 8,--27, 28, 30) this communicates with the general external surface by the small opening at the extremity of the p.e.n.i.s, and in the female by the opening into the v.a.g.i.n.a. In its entirety it consists in a surface of wide extent, comprising in the male the urethra, a long ca.n.a.l which opens into the bladder, and is continuous with ducts that lead into the genital glands or t.e.s.t.i.c.l.es. The internal surface of the bladder is extended by means of two long tubes, the ureters, into the kidneys, and receives the fluid formed in these organs. In the female (Fig 9) there is a shallow external orifice which is continued into the bladder by a short ca.n.a.l, the urethra, the remaining urinary surface being the same as in the male; the external opening also is extended into the short, wide tube of the v.a.g.i.n.a, which is continuous with the ca.n.a.l of the uterus. This ca.n.a.l is continued on both sides into the Fallopian tubes or oviducts. There is thus in the female a more complete separation of the urinary and the genital surfaces than in the male. Practically all of the waste material of the body which results from cell activity and is pa.s.sed from the cells into the fluid about them is brought by the blood to the kidneys, and removed by these from the blood, leaving the body as urine.

[Ill.u.s.tration: FIG. 8.--A LONGITUDINAL SECTION THROUGH THE MIDDLE OF THE BODY SHOWING THE EXTERNAL AND INTERNAL SURFACES AND THE ORGANS.

1. The skull.

2. The brain, showing the convolutions of the gray exterior in which the nerve cells are most numerous.

3. The white matter in the interior of the brain formed of nerve fibres which connect the various parts of this.

4. The small brain or cerebellum.

5. The interior of the nose. Notice the nearness of the upper part of this cavity to the brain.

6. The hard or bony palate forming the roof of the mouth.

7. The soft palate which hangs as a curtain between the mouth and the pharynx.

8. The mouth cavity.

9. The tongue.

10. The beginning of the gullet or oesophagus.

11. The larynx.

12. The windpipe or trachea.

13. The oesophagus.

14. The thyroid gland.

15. The thymus gland or sweetbread.

16. The large vein, vena cava, which conveys the blood from the brain and upper body into the heart.

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