A, a diagrammatic section of the human body taken vertically through the median plane. C.S. the cerebro-spinal nervous system; N, the cavity of the nose; M, that of the mouth; Al., Al. the alimentary canal represented as a simple straight tube; H, the heart; D, the diaphragm; Sy. the sympathetic ganglia.

B, a transverse vertical section of the head taken parallel with the line a b; letters as before.

C, a transverse section taken along the line cd; letters as before.

Transverse sections, taken perpendicularly to the axis of the vertebral column, or to that of the skull, show still more clearly that this is the fundamental structure of the human body, and that the great apparent difference between the head and the trunk is due to the different size of the dorsal cavity relatively to the ventral. In the head the former cavity is very large in proportion to the size of the latter (Fig. 1, B); in the thorax, or abdomen, it is very small (Fig. 1, C).

The limbs contain no such chambers as are found in the body and the head; but, with the exception of certain branching tubes filled with fluid, which are called blood-vessels and lymphatics, are solid, or semi-solid, throughout.

11. Such being the general character and arrangement of the parts of the human body, it will next be well to consider into what constituents it may be separated by the aid of no better means of discrimination than the eye and the anatomist's knife.

With no more elaborate aids than these, it becomes easy to separate that tough membrane which invests the whole body, and is called the skin, or integument, from the parts which lie beneath it. Furthermore, it is readily enough ascertained that this integument consists of two portions: a superficial layer, which is constantly being shed in the form of powder or scales, composed of minute particles of horny matter, and is called the epidermis; and a deeper part, the dermis, which is dense and fibrous. The epidermis, if wounded, neither gives rise to pain nor bleeds. The dermis, under like circumstances, is very tender, and bleeds freely. A practical distinction is drawn between the two in shaving, in the course of which operation the razor

ought to cut only epidermic structures; for if it go a shade deeper, it gives rise to pain and effusion of blood.

The skin can be readily enough removed from all parts of the exterior, but at the margins of the apertures of the body it seems to stop, and to be replaced by a layer which is much redder, more sensitive, bleeds more readily, and is rendered moist. by giving out a more or less tenacious fluid, called mucus. Hence, at these apertures, the integument is said to stop, and to be replaced by mucous membrane, which lines all those interior cavities, such as the alimentary canal, into which the apertures open. But, in truth, the integument does not come to an end at these points, but is directly continued into the mucous membrane, which last is simply an integument of greater delicacy, but consisting fundamentally of the same two layers, a deep, fibrous, sanguine, and nervous layer, and a superficial, horny, insensible, and bloodless one, now called the epithelium. Thus every part of the body might be said to be contained between the walls of a double bag, formed by the epidermis, which invests the outside of the body, and the epithelium, its continuation, which lines the internal cavities.

12. The dermis, and the deep, sanguine layer, which answers to it in the mucous membranes, are chiefly made up of a filamentous substance, which yields abundant gelatine on being boiled, and is the matter which tans when hide is made into leather. This is called areolar, fibrous, or, better, connective tissue.* The last name is the best, because this tissue is the great connecting medium by which the

*

Every such constituent of the body, as epidermis, cartilage, or muscle, is called a "tissue."

different parts of the body are held together. Thus it passes from the dermis between all the other organs, ensheathing the muscles, coating the bones and cartilages, and eventually reaching and entering into the mucous membranes. And so completely and thoroughly does the connective tissue permeate almost all parts of the body, that if every other tissue could be dissected away, a complete model of all the organs would be left composed of this tissue. Connective tissue varies very much in character; sometimes being very soft and tender, at others as in the tendons and ligaments, which are almost wholly composed of it-attaining great strength and density.

13. Among the most important of the tissues imbedded in and ensheathed by the connective tissue, are some the presence and action of which can be readily determined during life.

If the upper arm of a man whose arm is stretched out be tightly grasped by another person, the latter, as the man bends up his fore-arm, will feel a great soft mass which lies at the fore part of the upper arm, swell, harden, and become prominent. As the arm is extended again, the swelling and hardness vanish.

On removing the skin, the body which thus changes its configuration is found to be a mass of red flesh, sheathed in connective tissue. The sheath is continued at each end into a tendon, by which the muscle is attached, on the one hand, to the shoulder- · bone, and, on the other, to one of the bones of the fore-arm. This mass of flesh is the muscle called biceps, and it has the peculiar property of changing its dimensions-shortening and becoming thick in proportion to its decrease in length-when influenced

*

It is

by the will as well as by some other causes. by reason of this property that muscular tissue becomes the great motor agent of the body; the muscles being so disposed between the systems of levers which support the body, that their shortening necessitates the motion of one lever upon another.

14. These levers form part of the system of hard tissues which constitute the skeleton. The softer of these are the cartilages, composed of a dense, firm substance, ordinarily known as "gristle." The harder are the bones, which are masses either of cartilage, or of connective tissue, hardened by being impregnated with phosphate and carbonate of lime. They are animal tissues which have become, in a manner, naturally petrified; and when the salts of lime are extracted, as they may be, by the action of acids, a model of the bone in soft and flexible animal matter remains.

More than 200 separate bones are ordinarily reckoned in the human body, though the actual number of distinct bones varies at different periods of life, many bones which are separate in youth becoming united together in old age. Thus there are originally, as we have seen, thirty-three separate bodies of vertebræ in the spinal column, and the upper twenty-four of these commonly remain distinct throughout life. But the twenty-fifth, twentysixth, twenty-seventh, twenty-eighth, and twentyninth early unite into one great bone, called the sacrum; and the four remaining vertebræ often run into one bony mass called the coccyx. In early adult life, the skull contains twenty-two naturally separate bones, but in youth the number is much greater,

* Such causes are called stimuli.