3. Respiration, or the exposure of the nutritive fluid to the action of the atmosphere.

4. Circulation, or the distribution of the converted matter to every part of the animal, for its repair and augmentation. The process is named circulation, from the mode in which it is carried on in the generality of animals.

5. Secretion, or the separation, and deposition of the particles composing the structure of animals and vegetables, as well as the formation of various substances which they produce from the circulating fluids.

6. Irritability, or the principle by which living fibres contract, by means of which absorption and circulation are carried on, and which is more strikingly manifested by the occasional exertions of the muscular powers.

7. Sensation, by which animals become conscious of their own existence, and of that of external bodies.

8. Generation, by which new beings, similar to the parents, are formed and produced.

PARTICULAR ANATOMICAL DESCRIPTION OF THE HUMAN BODY.

After a cursory notice of the cellular substance, which forms the grand uniting medium of the various structures in the body, ard of membranes, which are formed of that substance, we shall proceed to describe the other parts, chiefly according to the technical arrangement above mentioned.

Cellular substance, or cellular membrane, tela cellulosa or mucosa of Latin writers, is the medium which connects and supports all the various parts and structures of the body. Any person may gain a general notion of this substance, by observing it in joints of veal, where it is always inflated by the butchers. It consists of an assemblage of fibres and laminæ of animal matter, connected to each other so as to form innumerable cells or small cavities, from which its name of cellular is derived. It pervades every part of the animal structure. joining together the minute fibrils of muscle, tendon, or nerve, it forms obvious and visible fibres; it collects these fibres into large-fasciculi; and by joining such fasciculi or bundles to each other, constitutes an entire muscle, tendon, or nerve. It joins together the individual muscles, and is collected in their intervals. It surrounds each vessel and nerve in the body; often connecting these parts together by a firm

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kind of capsule, and in a looser form joining them to the neighbouring muscles, &c. When condensed into a firm and compact structure, it constitutes the various membranes of the body, which, by long maceration in water, may be resolved into a loose cellular texture. Its general condensation on the surface of the body constitutes the cutis, or true skin, which is, in fact, a membrane. In the bones it forms the basis or ground-work of their fabric, a receptacle, in the interstices of which the earth of bone is deposited. As cellular substance is entirely soluble in boiling water, it is ascribed by chemists to that peculiar modification of animal matter termed gelatine. In consequence of its solution by the united agencies of heat and moisture, the muscular fibres separate from each other, and form the other structures of the body. This effect is seen in meat which is subjected to long boiling or stewing for the table, or indeed in a joint which is merely overboiled.

Its watery solution assumes, when cold, the appearance of jelly; and, after a particular mode of preparation, constitutes glue.

The interstices of the cellular substance are lubricated and moistened by a serous or watery fluid, poured out by the exhalant arteries, and again taken in by the lymphatics. It thus acquires a pliancy and softness, which adapt it particularly to serve as a connecting medium for parts, which have motion on each other. The importance of this property will be best understood by observing the effects of its loss. Inflammation or abscess often causes an induration or consolidation of the cellular texture, by which the integuments are fixed to the muscles, the muscles are firmly united to each other, and to the surrounding parts, and the motions of the whole are considerably im paired.

From the universal extent of this cellular texture, two conclusions may be drawn ; 1st, it forms the basis of the whole animal fabric, in such a way, that if we conceive every part removed but this, the form of the whole would still be expressed in cellular substance; 2ndly, it forms a connection and passage between all parts of the body, however remote in situation, or dissimilar in structure. For the cells of this substance every where communicate; as we may collect from facts of the most common and familiar occurrence. In emphysema, where air escapes from the lung wounded by a broken rib, into the cellular

substance, it spreads rapidly from the chest into the most remote parts of the body; and has even been known to gain admission into the eye-ball. A similar diffusion of this fluid may be effected by artificial inflation, which is commonly practised by butchers on the carcases of calves. In anasarca, or preternatural accumulation of fluid in the cellular substance, the most depending parts are the most loaded; and punctures in these drain the water off from the whole body. Adipous substance, or fat.-The cells of the cellular substance, in many parts of the body, are destined for the reception of a fluid, termed fat. This is of an unctuous nature, inflammable, lighter than water, usually inodorous, and, generally speaking, similar to the vegetable oils. It is white in young animals, and becomes yellower as they advance in age: this difference may be seen in the carcases of a calf and cow. It is always more or less fluid in the living subject; in carnivorous animals, and in man, it retains much of its oily appearance after death; but in herbivorous animals it eonstantly assumes a concrete form. Dr. Hunter called those parts of the cellular substance, which contain fat, adipous cellular substance; and distinguished the other by the epithet reticular.

As the fat is deposited in cells, it assumes in general a kind of granular form. It varies considerably in consistence. That of the orbit is the softest in the body, and forms a well-known epicurean bonne bouche, in a boiled calf's head. The fat about the kidneys becomes particularly hard after death, and is called suet. The globules or portions of this are very large, and it contains on the whole less cellular substance than any fat in the 'body. There is generally a layer of fat under the skin; whence a membrana adiposa has been sometimes enumerated as one of the common integu ments of the body.

Some parts of the body never contain fat, even in subjects who have the greatest accumulation of this fluid. This is the case with the scrotum, the integuments of the penis, and the eyelids: it is obvious that the functions of these parts would be com pletely destroyed, if they were subject to the enormous accumulations of fat, which occur in other parts of the body. Several of the viscera also never contain any fat, probably for the same reason; this is the case with the brain and lungs.

The quantity of fat varies according to the age, the state of health, and the pecu

liar habit or disposition of the individual. It is not found in the early periods of fœtal existence; and cannot be distinguished with any certainty sooner than the fifth month after conception.

In the fœtus, and for some time after birth, the fat is confined to the surface of the body, and is only found in a stratum under the skin. It begins, however, gradually to be deposited in the intervals of the muscles, and on the surface of some viscera. In old subjects, however thin they may seem on an external view, there is always much fat, penetrating even the substance of the muscles: the bones are greasy throughout; the heart is more or less loaded, as are also the parts in the abdomen.

There is a considerable difference in the quantity of fat in different individuals; and in some there is a propensity or disposition to its accumulation; a sedentary life, copious food, and tranquil state of the mind, are particularly favourable to the increase of fat, which sometimes proceeds to such a pitch, from the continuance of these causes, that it must be considered as a disease, and is attended with the greatest inconvenience to the individual. General diseases of the frame are commonly attended with an absorption of the fat from the cellular substance: acute disorders cause a very rapid emaciation. In no case is the adipous substance more completely removed from the whole body than in anasarca, where its place is supplied by a serous fluid.

The uses of the fat seem to be, in part, common to it with the cellular substance: it connects contiguous parts, and at the same time prevents their coalition. It admits of their moving on each other withi freedom and facility. Its deposition under the integuments gives a roundness and con vexity to the surface, on which the beauty of the human form principally depends. Indeed, its accumulation in particular situations immediately influences the outline of the part; as in the orbit, the cheek, and the buttocks. The effects of its loss is most disagreeably manifested in the lank cheek and hollow eye of an emaciated patient.

It has been supposed that the fat absorbed under certain circumstances is applied to the nutrition of the body; as in hybernating animals.

Membrane-In the foregoing observafions on cellular substance, we have stated that membranes are formed by a condensation of that substance. They consist of

thin sheets of compacted and close cellular texture. This is proved by long maceration in water. The fluid gradually penetrates the interstices, and resolves the membrane into a loose and floculent substance. They are found in every variety of density and softness. A grand use of membranes is to line what anatomists call the circumscribed cavities of the body. These are hollow spaces, containing the different viscera, and in every instance completely and accurately filled by such viscera; so that the term cavity, when used by anatomists, does not, as in common language, denote a void or empty space.

Membranes have a smooth internal polished surface, turned towards the contained viscera. This is constantly moistened by a lubricating fluid exhaled by the minute arteries of the part, and bestows on the surface of the membrane the greatest softness and smoothness. Hence the motions of the viscera are performed with perfect facility, and they are prevented from adhering to each other, or to the sides of the containing cavity. The extent of such cavities is bounded and defined by the lining membranes, and hence arises the epithet circumscribed. To increase the facility of motion, the surface of the contained viscera is covered by productions of the same membrane, and always therefore possesses the same smoothness and polish with the sides of the cavity. In the carcase of an animal just slaughtered, the lubricating secretion flies off in the form of a fine vapour, when the cavity of the belly or chest is laid open. It is nothing more than an increase of this natural secretion, combined perhaps with a deficient absorption, that gives rise to dropsies of the different cavities.

The opposite or external surface of the membrane is rough and cellular; and adheres to the various parts, which form the sides of the cavity.

Another use of membranes is to form blood-vessels, or tubes, for conveying the nutritious fluid to all parts of the body. The bore or hollow of the tube is perfectly smooth and polished, so that the blood experiences no obstruction in its course; and the external surface is rough, to connect it with the surrounding parts. In a similar manner are formed the stomach and intestines, which receive the food; the urinary bladder, which holds the urine, &c.

It must be obvious, that for all the purposes which we have enumerated, whether for lining circumscribed cavities, for conveying the blood, for receiving the food, or

holding any other liquors, it is essentially necessary that membranes should be impermeable to fluids in the living state.

OSTEOLOGY.

The bones are the most solid parts of the body. They are composed of a vascular substance, not differing materially in structure from that of the rest of the body, except that there is deposited in its interstices an earthy matter, which gives to the whole mass rigidity, strength, and a permanent figure. The nutrient vessels of arteries, membranes, and ligaments, occasionally deposit lime, and cause the ossification of those parts.

The account of the original formation of the bones in the fetus, is technically termed osteogeny. The parts of the young fœtus, which are afterwards to become bones, are at first cartilaginous; and their substance is rendered white and firm in proportion to the quantity of lime deposited in it. The quantity at the time of birth is only sufficient to give firmness to the whole mass, not to prevent its flexibility.

The extremities of all the long bones consist of large portions of cartilage, and these, by degrees become bony. The formation of bone begins in the centre of the cartilage, and gradually extends from thence to the remote parts, so that the separate piece of bone, formed at the extremity, remains till near the time of puberty, conjoined to the body of the bone by a crust of cartilage. In this state it is technically termed an epiphysis. The body, or middle part of the bone, is called the diaphysis. The projecting parts, or processes of bones, are also in many instances originally epiphyses. The time by which these epiphyses are consolidated by a bony union with the diaphysis, varies in different bones, but it is not prolonged in any much beyond the age of puberty.

We perceive an evident advantage in the bones of the fœtus being formed as they are. Their flexibility admits of the form of the limbs becoming adapted to the varying figure of the pelvis, through which they must pass; and their elasticity, which is powerful, restores them afterwards to their natural shape.

The animal substance contained in bones is demonstrated by immersion in weak acids, which dissolve the earth, and leave a kind of cartilage similar to that in which the bone was originally formed. Long boiling in a close vessel removes the gelatinous sub

stance, which is dissolved in the water. The earth of bones is demonstrated by calcination, which drives off the animal matter, and leaves the earth alone behind. This earth consists chiefly of phosphate of lime; but there is also a small proportion of carbonate of lime. In young subjects the animal substance predominates; and the bone appears redder, in consequence of the arteries being larger and more numerous. The bones of old persons contain more earth, and are consequently whiter and less vascular.

Some recent experiments have shewn the quantity of jelly contained in bones to be much larger than was supposed, and as it forms a very good soup when dissolved in water, the circumstance is of considerable importance, as furnishing an article capable of supplying much wholesome nutriment. The quantity of soup furnished from a given bulk of bruised or pounded bones, boiled in a vessel with a closed lid, considerably exceeds that which can be extracted from the same quantity of meat. Of course the articular heads of bones, and the reticular texture, in general furnish the greatest quantity.

It has been generally taught, that bones are composed of fibres and lamina: the fact is that they consist of a reticulated texture, very similar to cellular substance in other parts of the body.

According to the obvious differences in their forms, bones are divided into the long

and flat.

Two kinds of structure may be observed in all bones: in the one, the bony substance is condensed, and leaves no interstices; in the other, there is a mere net-work of bony fibres and plates, leaving numerous intervals. The latter is termed the cancellous substance of bones.

The cylinder of a long bone is composed entirely of the firmer substance, and in its centre is hollowed out to contain the marrow. In those extremities of the bones, which form the joints, which are greatly expanded, in order to increase the extent of surface, there is a thin layer of the compact substance, but all the interior is cancellous. In broad or flat bones, the firmer substance is formed into two plates or tables, and the interval between these is occupied by cancelli.

Many advantages arise from this arrangement of the earth of bones. The long bones are made slender in the middle, to allow of the convenient collocation of the large muscles around them; they become expanded VOL. I.

at their extremities, to afford an extent of surface for the formation of joints, and the support of the weight of the body. A cavity is left in the middle; for if all the earthy matter had been compacted into the smallest possible space, the bones would have been such slender stems, as to be very unsuitable to their offices, and if they had been of their present dimensions, and solid throughout, they would have been unnecessarily strong and weighty.

The phenomena, which result from feeding an animal with madder, sufficiently demonstrate the existence of blood-vessels and absorbents in the bones. There is a strong attraction between the earth of bone and the colouring matter; by means of which they unite and form a beautiful red substance. The whole of the bones of an animal assume this colour soon after an animal has been taking the madder. If it be left off, the bones in a short time resume their natural white appearance, from the absorption of the red colouring substance. The short time in which growing bones become thoroughly died, and in which again the preternatural tint is lost, prove that even in these, the hardest parts of our frames, there is a process of removal of old parts, and deposition of new ones constantly going on.

That bones possess nerves, as well as arteries, veins, and absorbents, cannot be doubted. Although in the natural state they seem to be insensible, they become extremely painful when diseased; and again, a fungus which is sensible sometimes grows out of a bone, though it may have no connexion whatever with the surrounding soft parts; of course it must have derived its nerves, by means of which it possesses sensation, from the bone out of which it arose.

Bones are covered by a strong and firm membrane, termed periosteum, on which the vessels are first distributed; from this they descend into the substance of the bone. The vessels enter through holes, which are evident on the surface, and which are larger and more numerous in the extremities of the long bones than in the middle.