axis (Fig. C.S., C.S.). The ventral chamber, or cavity of the face, is almost entirely occupied by the mouth and pharynx, into which last the upper end of the alimentary canal (called gullet or œsophagus) opens.

10. Thus, the study of a longitudinal section shows us that the human body is a double tube, the two tubes being completely separated by the spinal column and the bony axis of the skull, which form the floor of the one tube and the roof of the other. The dorsal tube contains the cerebro-spinal axis; the ventral tube contains the alimentary canal, the sympathetic nervous system, the heart, and the lungs, besides other organs.

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 bloodvessels and lymphatics, are solid or semi-solid, throughout.

II. 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 the deeper part, the dermis, which is dense and fibrous (Fig. 32). 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 bleeding.

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 which keeps itself continually moist by giving out a more or less tenacious fluid, called mucus. Hence, at these apertures, the skin 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 skin does not really 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 layer, containing blood-vessels, and a superficial 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 alimentary canal and similar 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 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

I Every such constituent of the body, as epidermis, cartilage, or muscle, is called a "tissue." (See Lesson XII.)

much in character; in some places 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 former 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 by the will as well as by some other causes,1 and of returning to its original form when let alone. This temporary change in the dimensions of a muscle, this shortening and becoming thick, is spoken of as its contraction. It is 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 contraction necessitates the motion of one lever upon another.

14. These levers form part of the system of hard tissues which constitute the skeleton. The less hard of these are the cartilages, composed of a dense, firm substance, ordinarily known as "gristle." The harder are the bones, which are masses of tissue allied to cartilage, or to 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

I Such causes are called stimuli.

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, twenty-sixth, twentyseventh, twenty-eighth, and twenty-ninth 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, and in old age far less. Twenty-four ribs bound the chest laterally, twelve on each side, and most of them are connected by cartilages with the breast-bone. In the girdle which supports the shoulder, two bones are always distinguishable as the scapula and the clavicle. The pelvis, to which the legs are attached, consists of two separate bones called the ossa innominata in the adult; but each os innominatum is separable into three (called pubis, ischium, and ilium) in the young. There are thirty bones in each of the arms, and the same number in each of the legs, counting the patella, or knee pan.

one

All these bones are fastened together by ligaments, or by cartilages; and where they play freely over another, a coat of cartilage furnishes the surfaces which come into contact. The cartilages which thus form part of a joint are called articular cartilages, and their free surfaces, by which they rub against each other, are lined by a delicate synovial membrane, which secretes a lubricating fluid-the synovia.

15. Though the bones of the skeleton are all strongly enough connected together by ligaments and cartilages, the joints play so freely, and the centre of gravity of the body, when erect, is so high up, that it is impossible to make a skeleton or a dead body support itself in the upright position. That position, easy as it seems, is the result of the contraction of a multitude of muscles which oppose and balance one another. Thus, the foot affording

the surface of support, the muscles of the calf (Fig. 2, I) must contract, or the legs and body would fall forward.

FIG. 2.-A DIAGRAM ILLUSTRATING THE ATTACHMENTS OF Some of the MOST IMPORTANT MUSCLES WHICH KEEP THE BODY IN THE ERECT POSTURE,

I. The muscles of the calf. II. Those of the back of the thigh. III. Those of the sp.ne. These tend to keep the body from falling forward.

1. The muscles of the front of the leg. 2 Those of the front of the thigh. 3. Those of the front of the abdomen. 4, 5 Those of the front of the neck. These tend to keep the body from falling backwards. The arrows indicate

the direction of action of the muscles, the foot being fixed.