The skin. Cuticle. True skin. Papillæ. FIG. 58. b 180. The skin, while it is an extensive excreting organ, performs other important offices. It serves as a firm yet very flexible and soft covering to the body, protecting its internal parts from injury. It is highly endowed with nerves for two purposes-the one, that it may act as a sentinel to warn of danger; and the other, that it may be the seat of the sense of touch. That you may see how well it is fitted to perform these various functions, I will describe here its structure. What is very commonly spoken of as the skin, is not really the skin, but only a covering for it. When the skin is rubbed off, as it is expressed, it is only this covering of the skin, or cuticle, which is removed. The skin which is raised by a blister is this cuticle. The great object of the cuticle is to protect the true skin, which is very highly endowed with nerves for the purposes mentioned above, and which therefore, if uncovered, would prove a source of severe suffering. As it is, the cuticle protects the skin effectually, and yet does not interfere with its functions as the organ of the sense of touch. It is of so slight and so soft a texture, that the nerves of touch may readily receive impressions through it. It is composed, as you will see in the next chapter, of many layers of minute round cells, the outermost layers being made up of these cells broken, and emptied of the fluid which they contained. The true skin, which the cuticle covers, is of a fibrous texture, with a good supply of both nerves and blood vessels. On the surface of this true skin next to the cuticle are eminences called papillæ. In these are seated the extremities of the nerves of touch. Fig. 58 represents a highly magnified section of a bit of the skin from Vertical section of the SOLE OF THE FOOT. Tubing in the skin. Insensible perspiration. Sebaceous glands. the sole of the foot; a is the cuticle; c is the true skin; b represents the papillæ. You observe that the deepest layers of the cuticle, next to these papillæ, are more colored than the outer ones. The coloring matter of the skin is situated here. You observe also a tube which runs up through the cutis or true skin and the cuticle, and in the latter part of its course has a sort of cork-screw arrangement. This is the discharging tube of the sweat-gland, d, lying within the true skin, and surrounded with globules of fat. These glands are more numerous in some parts of the skin than in others. They are particularly numerous on the palms of the hands, and on the soles of the feet. Mr. E. Wilson counted, with the aid of the microscope, 3528 of them in a square inch on the palm of the hand. Reckoning the length of one of these at one quarter of an inch it gives 882 inches or 73 feet of tubing in this small space. He calculated the amount of this tubing in the skin of the whole body as being 48,600 yards, or nearly 28 miles. The amount of excretion from the seven millions of these tubes, which open on the surface of the skin, is very great. Many experiments have been tried to determine what the amount is in 24 hours, but approximations only to the truth, of course, could be obtained, and the results of the experiments have differed much. While the excretion is great in amount, it is very important. It is, as you have seen in the chapter on Respiration, a great means of regulating the temperature of the body. It is also the means of discharging from the body a portion of its waste. This waste is dissolved in or mingled with the water or vapor of the perspiration. The perspiration is ordinarily insensible, as it is termed; that is, it is in the form of vapor. But sometimes, as in vigorous exercise, when the sweat glands are rendered very active, chiefly to prevent too great an accumulation of heat, the perspiration becomes sensible. most 181. There is another set of glands in the skin called sebaceous glands, which secrete an oily fluid. They have also thin tubes like the sweat glands. They are abundant where the skin specially needs an oily lubrication, as where there are folds in the skin or hairs, or where the skin is exposed to friction, or to the drying atmosphere. They are very abundant on the face and head. The amount of the oily secretion of these glands is very great in the skin of races fitted to inhabit warm climates. Every hair has sebaceous glands connected with it, as represented in Fig. 59; in which 6 is the hair emerging from the skin; Influence of labor on wear and tear, and on absorption. a a are the sebaceous glands pouring their secretion by thin tubes into the tube or canal in which the hair grows; c the root of the hair surrounded with fat globules. From all this you see that the skin, with its two sets of glands and tubes, its nervous papillæ, and its layers of constantly renewed cells, making the cuticle, is a complicated organ, and is thus fitted to perform its functions as an organ of sensation, and at the same time of excretion, while it is also a pliable but firm covering for the body. a FIG. 59. HAIR and sebaceous glands. 182. You have seen in the facts developed in this chapter, that there is constant change going on in all parts of the body. Particles which have become useless are taken up by the absorbents, while the formative vessels deposit others to take their places. The rapidity with which this change occurs, depends mostly upon the activity of the individual. The busy laborer, whether the labor be bodily or mental, requires more nourishment than the indolent man, because there is more waste in his case, from the wear and tear occasioned by motion or thought, and there is therefore a necessity for a larger supply of repairing material. The difference, it is true, is not as great in regard to mental labor, as in regard to that of the body; but still it is very apparent. This dependence of the amount of change in the system upon the degree of activity is very manifest, if we compare different animals together in this respect. I have already contrasted the frog and the canary bird in regard to respiration (§ 156,) and they can be contrasted in this respect also. As the frog makes but little exertion either of body or mind, there is but little change in his body, and but little nutriment is required to supply the small waste that occurs. But in the ever active canary there is much waste from this action, and therefore there must be much eating to supply the material of repair. As he sings and hops from perch to perch, his mind as well as his body is vastly more active than that of the frog; and so the particles in his brain and nerves, as well as in his muscles, are oftener changed. You see the same thing still more strikingly, if you contrast Change varies in different parts of the body, and in the same body at different times. the torpid state of the hibernating animal in winter, with his active state in the warm weather. In his torpid state life is dormant, almost at a stand still, sometimes entirely so. And the more perfect the quiescence, the less is the change, and the less, therefore, the need of nutrition. The fat which he lays up in the autumn (§ 65) answers all his necessities both for nutrition and for heat. 183. The proportion, thus seen to exist between the amount of change and the degree of activity, is exemplified in a comparison between different parts of the body. In those which are most actively used the change of decay and repair is going on most constantly. The active muscles and nerves are continually changing; while the bones, which are only passive instruments of motion are changed very slowly. And it is a significant fact, that in the case of the muscles and nerves, the waste particles are to a large extent of the entirely useless kind (§ 178), for they are mostly absorbed by the veins, there being in them but few lymphatics. That is, whenever we think, or feel, or move, we render entirely useless quantities of the particles which make up the structure of the muscular and nervous systems, and these are got rid of at the proper outlets, while other particles immediately take their places. 184. It is a very prevalent notion in the community, that the human body changes throughout once in every seven years. But you have seen that the change is very unequal in different parts of the body, and is dependent to a great extent on circumstances. Sometimes very rapid changes occur. Thus, when one has been much reduced by sickness, and then on recovery quickly regains his usual bulk, the body is very extensively changed in a short period of time. Ordinarily the circumstance which most influences the change is, as you have seen, the degree of activity which exists, whether we look at an animal as a whole, or at the tissues separately. 185. In this constant change going on in the body, life and death may be said to be brought into very near companionship. Every act of the mind, and every movement of the body breaks down some of the structure; and the particles, which are no longer fitted to maintain the living functions, must be taken away as refuse dead matter, and new particles endowed with vital affinities must take their place. Action, destruction, repair, are the successive events which are ever occurring in in every part of our frame. Action is followed by destruction, and in proportion to its intensity; and repair is necessary to The formative vessels shown by the microscope to be cells. fit for further action. And so through life the nutritive functions are thus struggling against the tendency to decay and death, till at length at the appointed limit the struggle is given over, the vital affinities release their hold, the common laws of dead matter take possession of the body, and the soul passes to a world where decay and change are unknown. CHAPTER IX, 186. In previous chapters, in treating of the construction of the body, I have spoken of the formative vessels in accordance with the common language of physiologists. The common idea has been hitherto, that the work of construction is performed by vessels appended to the capillaries. The capillaries were considered as the repositories of the blood, they receiving it from the arteries, and holding it in readiness for the use to which it is to be put by the formative vessels. These formative vessels, it was supposed, exercised in some way a power of selection in regard to the constituents of the blood, and also a power of uniting the constituents thus chosen into particular forms. In this way physiologists accounted for the formation of all the different structures in the body. What shape these formative vessels had, or how they were arranged no one pretended to know. But of their existence no one had a doubt, for there seemed to be an absolute necessity for supposing some apparatus of vessels appended to the capillaries for the performance of this function. 187. But the microscope has of late years revealed phenomena which have changed our ideas on this subject, and which must to some extent change our modes of expression in relation to it also. It has showed us agencies which differ from those which we had supposed to exist. The subject is an interesting one, and I propose in this chapter to give you some glimpses of this interior life, as it may be termed, of the body. 188. It is found by the aid of the microscope, that all the minute operations of the system are performed by the agency of cells. They are not such cells as I described in § 64 as existing in the cellular tissue, which are mere interstices, communicating together. But they are bladders or sacs, and are |