1. Cuticle.

2. Rete mucosum.

3. Corpus papillare.

4. Cutis vera.

5. Cellular membrane.

6. Paniculus carnosus.

There are in certain parts of the body, and especially in animals, muscular fibres passing up through the cellular membrane, and inserted obliquely into the cutis vera, as at 6 in the plate. These form the muscular web, or paniculus carnosus. It is this which raises the feathers in birds and which in the hedgehog and porcupine rolls up the body, and erects the spines; and by means of this, animals shake off flies from their skin.

28. The skin is defended from the action of moisture, and is also kept soft and pliant by an oily fluid, which is separated from the blood by numerous sebaceous follicles. We find these the most numerous where there are folds of the skin, or hairs, or where the surface is exposed to friction. We sometimes see the oily matter secreted by these glands assume a dark appearance, in the form of black specks, scattered over the face, which are called by many worms, because when pressed out they assume that aspect.

29. The hair is usually treated of by physiologists as an appendage to the skin. The hair takes its origin from the cellular membrane in the form of bulbs, each bulb consisting of two parts; an outer, which is vascular, and from which the hair obtains its nourishment; an inner, which is membraneous, and which forms a sheath or tube to the hair during its passage through the skin. The external covering of each hair is of a horny structure, while the central part is

soft and pulpy, and is called medulla, or pith.) The colour of the hair varies in different individuals, and is generally supposed to depend on the fluids contained in the pith. The hair, it is said, has been known in several instances to have changed from black to grey in the course of a single night, from the effects of grief, fear, or some other great mental agitation. Vauquelin supposes that as the colouring matter of the hair is destroyed by acids, this phenomenon is owing to the production of some acid in the system. Others suppose that the effect depends upon the sudden stagnation of the vessels which secrete the colouring matter, while the absorbents continue to act and remove that which already exists. There are probably two causes which act in changing the hair to grey; (the first is a defective secretion of a colouring fluid, and the second, the canals which convey the fluid into the hair become obliterated In the first case, the hair will remain; in the last, it inevitably dies and drops

out.

6

30. The figure on the last page represents the pulp of a hair injected; 1, cut surface of hair; 2, the pulp; 3, injected vessel ramifying in it. The figure on the right shows the hair in its fallicle, though the drawing was made from the whisker of a Walrus; 2, the cutis; 3, external sheath of the fallicle; 4, internal sheath continuous with the cuticle ; 5, pulp; 6, shaft of hair; 7, large nerve going to it.

31. The nails are also an appendage to the skin. Their chief use seems to be to support the pulp of the finger while it is exercising touch. Animals are provided with structures analogous in their physical and vital properties, such as horns, beaks, hoofs, nails, spurs, scales, &c. Some physiologists consider the teeth as belonging to the same class. The various and peculiar functions of the skin, so important to be understood in order to the preservation of the health, will be fully described hereafter.

Questions. Of what is the human body composed? What is said of the identity of solids and fluids? What proportion do the fluids bear to the solids? How modified by age? What purposes do the fluids serve? What is said of the solids? How are their particles arranged to form the different organs? How are the areolae formed ? Of what shape is the ultimate animal solid? What is a primary tissue? How many primary tissues are there? What is a filament?-a fibre -a tissue-an organ-a function? Give an example? What is an apparatus?-a system ?-membrane? What are its uses? What is the simplest form of membrane ?-its use? What property does it possess? What its chemical composition? What remarkable circumstance connected with it? What is the seat of dropsy? What varieties of membrane are formed by the cellular tissue? Describe the adipose? What are serous membranes ?—where found ?—their use? structure? What are mucous membranes? Describe their structure, situation, use, &c. What is the skin a continuation of? Of how many parts is it composed? What is the external layer called?-what is its use? What is the layer next to the cuticle called? Of what is it the seat?—the next layer?-its use? Describe the cutis vera? What is it used for? How thick is the skin in man? By what contrivance is the skin susceptible of motion? How is the skin kept soft and pliant? What is said of the hair ?-what is its structure ?-what is its colour owing to? What causes it to turn grey? What is said of the nails?

CHAPTER IV.

STRUCTURE OF THE HUMAN BODY CONTINUED.

1. ANOTHER form of cellular structure is called fibrous, from the arrangement of its component parts, which consist of longitudinal fibres, plainly visible to the naked eye, placed parallel to each other and closely united. These fibres are sometimes so combined as to form a thin, smooth, dense, and strong membrane, like that which covers the external surface of bones, termed periosteum, or the internal surface of the skull, (dura mater.) At other times it forms a firm sheath, which dips down between the muscles, separating them from each other, or it may be spread over them, binding them firmly down in their places.

2. In the loins and along the back bone there is a strong binder of this kind, which greatly facilitates the motions of the body and powerfully contributes to the action of the muscles. Where this is weak, a belt round the waist is of considerable service, but in a well-formed person it is a clumsy substitute of art for nature, and ultimately tends to weaken the back, for exercise strengthens and invigorates all parts of the frame, while artificial substitutes invariably weaken the parts they are supposed to strengthen.

3. It also constitutes the strong, tough, and flexible band, used for tying parts firmly together, termed ligaments, such as are found about the joints, connecting the bones; and it also forms those white cords in which muscles terminate, called tendons, which serve to attach the muscles to the bones, thus acting as mechanical instruments of motion. This tissue forms a firm covering or envelope to the bones, muscles, tendons, cartilages, the eye, kidneys, spleen, and most of the organs of the body.

4. Every joint is enclosed by a firm fibrous bag, called the capsular ligament, the internal surface of which secretes a fluid, termed synovia, which lubricates the joints, and serves the same purpose that oil does in a piece of machinery [At the wrist and ankle, and along the fingers and toes, the ligaments form strong bandages to bind down the muscles and tendons, and enables them to act with greater precision and steadiness; otherwise when the muscle contracted, the tendons would start out like the string of a bow, and thus not only destroy the symmetry of the parts, but entirely prevent motion, When sprained by over exertion or accident, an artificial bandage becomes necessary; we then find how superior the natural ligaments are to any artificial bandage that can be contrived.

5. Tendon or Sinew.-This well-known structure is plainly perceptible at the wrist, on the back of the hand, at the heel, &c. They have a white, pearly lustre, possess great strength, but little elasticity. They are composed of bundles of parallel threads, bound together by transverse threads, and they vary both in figure and extent; some being cylindrical and tapering to the point of insertion, and in others forming broad expansions. So insensible are tendons, that when ruptured, as the tendon of Achilles at the heel, not the slightest pain is felt, but the person feels that he has received a smart blow, or that a part of the floor has given way under him.

6. Where force is to be exerted upon some distinct point, it is transmitted along a tendon, as forces in machinery are by ropes. When a tendon passes over a joint, a bone is formed sometimes where it crosses, as the knee-pan in front of the knee joint; which is a bone placed directly in the centre of the tendon, and serves, to throw the force farther from the centre of motion, thereby increasing the lever, and conse. quently the effect. Similar bones are often found in the tendons of the feet.