Two systems of nerves, cerepro-spinal and sympathetic.

know nothing in this world of a sentient and acting mind existing without a brain.

265. The system of nerves which we have been examining is termed the cerebro-spinal, from its two great central organs, the brain and spinal marrow. But there is another nervous system, the functions of which are involved in much mystery. It is called the system of the great sympathetic, or the sympathetic system. Sometimes it is called the nervous system of organic life, because it is so intimately and extensively connected with the nutritive processes; while the system that we have been considering is called the nervous system of animal life, because it regulates the functions peculiar to animals in distinction from plants, sensation, and spontaneous motion. While the sympathetic system is thus connected with the nutritive processes, it is also supposed to be the means of effecting the sympathetic connection between different parts of the body, and to act as the medium by which the passions and emotions of the mind produce their effects upon the functions of the different organs. In this system there are many ganglions or little brains, which communicate with each other by nerves. There is a chain of them along in front of the spinal column, and there are two quite large ones in the abdomen. This system has connections everywhere with the cerebro-spinal. The purposes aimed at in the particular arrangements of this system are as yet but little understood, and we probably never shall know as much about it as we shall about the cerebrospinal system. The arrangement of the sympathetic system differs very materially from the cerebro-spinal. It is a single system, and has no symmetrical arrangement, while the cerebro-spinal has throughout two halves which are precisely alike.

I have thus described the arrangements and functions of the nervous system to such an extent, as will prepare you for the consideration of those subordinate organs, by which the purposes of this system are accomplished. After treating of the organs of locomotion, the voice, and the senses, I shall call your attention again to this system, presenting some views of its uses and connections, which you will then be better prepared to understand.

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Bones the framework of the body. Composition of bone.

CHAPTER XI

THE BONES.

266. THE bones furnish the points of support and attachment for the muscles which move the different parts of the body. They are, therefore, the passive instruments of locomotion. I treat of the bones before the muscles, because you will then better understand the action and the arrangement of the muscles.

267. The bones, forming the framework of the body, not only furnish points of support and attachment to the muscles, but in many cases serve to defend important organs from injury. Thus, the soft brain is thoroughly secured from harm by being inclosed in the skull; and the lungs are surrounded by walls of bone so arranged, as you saw in the chapter on Respiration, that, while they defend the lungs from external violence, they secure a wide range of motion for the necessary expansion of these organs.

268. The bones are composed of two parts, the earthy or hard portion, and the animal portion which is soft. Each of these portions, as was stated in § 60, can be obtained separate from the other. These two portions of bone exist in different relative proportions in the different periods of life. In the child the animal portion predominates, while the mineral does in old age. It is a wise provision in regard to the child, for if his bones were as brittle as those of old age, or even as those of middle life, they would be often broken in the falls to which the child in its feebleness and carelessness is subjected.

269. There are some points of interest in relation to the structure of bone and its growth. I stated in § 61 that bone is generally formed in cartilage, the cartilage being formed first as a mould for the bone. Bone is deposited in two forms, solid and cellular. In the flat bones, as in the skull, the cellular structure lies between two plates of solid bone. In the long bones the cellular part is at the two ends, and is covered with a thin plate of solid bone, while the shaft is a hollow tube vith the bone very much condensed. This arrangement

Structure of bone. Long bones hollow. Marrow.

is seen in Fig. 83, representing the thigh-bone and the bone of the arm. Čertain well known mechanical principles are observed in this arrangement. The bone would be unnecessarily heavy if it were solid throughout. Lightness in a moving limb is of considerable importance. At the same time strength is to be carefully provided for in a bone which is to sustain the weight of the body, and to which the large muscles of the thigh are attached. By having the bone hollow, both of these objects, lightness and firmness, are secured. The principles involved are recognized by the architect in the construction of pillars, and we see them exemplified in the hollow stalks of plants. The hollow pillar has more strength than the same quantity of matter would have if in one compact mass; and the stalk which supports the full clusters of grain, would break under its load as it moves back and forth in the wind, if it were solid instead of being hollow. But the round cavity of the shaft of the bone does not extend to the ends. These are ne

FIG. 83.

cessarily large, in order to present broad surfaces for articulation with the neighboring bones; and strength and lightness are secured in this case by a cellular arrangement of the bony matter, the outer plate of solid bone being comparatively thin. There is obviously more firmness in the resistance to shocks or pressure, secured in this way, than there would be if the bony matter were all consolidated into a shell containing a cavity.

270. The round canal in the shaft and the cellular structure at the ends are filled with an oily substance called marrow. This, like all other fatty substances, is contained in fat cells, as described in the chapter on Cell-Life. The marrow is also present in the cellular structure between the plates of the flat bones. The cavities and the cells in bones have branching

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Mode of nutrition in bones. No blood vessels in their solid parts. about in them blood vessels, which are branches of arteries and veins that enter the body of the bone at some particular points, in the long ones near the middle of the shaft. It is from these blood vessels, together with those that come from the membrane investing the bone, called periosteum, that the bone is nourished. But, although an artery runs through the body of the bone, to branch out upon the walls of its cavity, none of its branches enter the very substance of the bone. How then is the bone nourished, that is, constructed and kept in repair? The manner in which the material for this purpose is carried to every point of the solid bone has been developed by the aid of the microscope, and I will describe it to you. If we cut across the solid portion of a bone, and examine it with a microscope, we see here and there orifices of certain minute canals that run lengthwise of the bone. These canals are found to communicate with the cavity of the bone and receive therefore blood, or some of the constituents of the blood, from the bloodvessels which are situated there. These orifices, as seen under the microscope, are represented in Fig. 84. Around these orifices a a, you see little dark spots arranged in rings, with lines running to them from the orifices. By magnifying the section of bone still more, we see what these spots and lines The dark spots are small cavities, and the lines are minute tubes running to them. In Fig. 85 is a representation of this arrangement as seen in a little portion of the section of bone, more highly magnified than it is in Fig. 84.

are.

FIG. 84.

The

No sensibility in bones. Variety of shape.

tubes pass out from the canals to the rows of cavities which are around the canals, and thus a circulation is kept up at every point of the solid bone. It is supposed that the blood itself does not circulate in these little channels and cavities in the solid bone, but a fluid containing the constituents of bone. For these channels are too small even to admit the cells which the microscope shows us as swimming in the blood. The fluid that circulates in them is selected from the blood, which is contained in the bloodvessels in the cavity of the bone, and in the periosteum that envelopes it.

271. It is a very common popular notion, that the bones are endowed with great sensibility, and especially the central part, the marrow. The surgeon is very often asked if the sawing of the bone in amputation is not very painful, and if when the saw reaches the marrow it does not produce agony. But the bones have in their healthy state no perceptible sensibility, as I have before stated, and the sawing of the bone in amputation occasions no suffering. When, however, a bone becomes inflamed, severe pain is one of the symptoms. And it is well that it is so; for if it were not, disease might go on to produce disastrous results in a part so covered up by others, without any warning of the danger of the case.

272. The bones are of every variety of shape, to suit the various offices which they are to fulfill. You will see this to be true, as you cast your eye over the skeleton as represented in Fig. 86. You first observe the somewhat round box of bones, which contains the brain, and at the same time furnishes sockets for the eyes, extended irregular surfaces for the apparatus of smelling, and for that of the taste, a place for the organs of hearing, and at its lower part, in connection with the lower jaw, a mill for grinding the food. Then you observe the many bones of the thorax or chest, containing and protecting the heart and the lungs. The spinal column, k, composed of twenty-four bones, you see as a firm but movable pillar, extending the whole length of the body, and having its base firmly planted upon that stout thick bone, the sacrum, which is wedged in so tightly like the key-stone of an arch, between the broad spreading bones on either side. To this pillar are strongly fastened the walls of the chest; and from the chest thus supported by the spine hang the lax front and lateral walls of the abdomen. Then below you see the pelvis, as it is called, a set of large bones so arranged in a bowl-form, as to offer a broad surface of support to the contents of the abdomen.