ANATOMY

modes of articulation; others, where a slight motion is required, being united by cartilage; and others, where extensive and varied motions are wanted, being connected by ligaments, membranes, or flesh — Ligaments are white, librous, glistening, and flexible substances, occurring in an infinite variety of forms and situations. They are, for the most part, exterior to the joint, and, by their great strength and trifling elasticity, preserve the relative position or connection of the bones in their various movements.— Membranes are thin, whitish webs or textures, more flexible and elastic than ligament. They not only a ist in the security and motion of joints, but fulfil a variety of other offices. They surround or line the cavities and the organs of the body, and contribute to unite and combine the whole; and, at the same time, interpose, and preserve a distinction, enabling separate parts either to co-operate or to act independently of each other. They vary in strength and texture, and (Mfferent terms are applied to them in different parts of the body: two within the skull are called malres; those which envelope muscular fibres are called aponeuroses; that which covers the lungs and lines the cavity of the chest is termed pleura; that which lines the cavity of the abdomen and its included viscera is named peritoneum; those which inclose articular surfaces are termed capsules; that which covers bone, periosteum; and, in other cases, they are called coats, or tunics. The remaining substance concerned in the connection of the bones is flesh: it is thus, that the upper extremities are connected with the body, and that many of the joints are rendered secure. But flesh performs another and more important office, inasmuch as it constitutes a principal part of the organs termed muscles, through the medium of which the various movements of the body are effected. Many of the muscles contain, besides flesh, a substance analogous to ligament, through the medium of which they are attached to the bones, and to which the term tendon is applied: muscles and tendons are composed of bundles of fibres, which may be unravelled to extreme minutene ; and when what appears to be a single fibre is viewed under the microscope, it resembles a chain of infinitely small globular particles. But though the muscles are the immediate organs of motion, they are dependent for their powers of contraction and relaxation upon the nerves with which they are supplied. These, when separately examined, appear in the form of white cords or threads; and, when traced to their origin, are found to i ue as it were from the brain, and. from its elongation, termed the spinal marrow. The trunks of the nerves are subdivided into branches, and these again into filaments, which enter into, and are, as it were, lost in the substance of the muscles and other organs of the body. Their functions are in some cases obedient to, and in others independent of the will: to the former belong the nerves of the locomotive muscles; to the latter, those of the heart, viscera, . When they are divided, the peculiar functions of the organs which they supply are impaired or impeded: thus, the muscles may be deprived of the power of contracting, the glands of secretion, the eye of sight, the ear of hearing, and the skin of feeling. The nervous trunks, which i ue in pairs from the brain or spinal marrow, amount to about forty; and in tracing them and their branches, they are found in certain different places to swell into knots, which are termed gawo'/^Ve, or they are reticularly aggregated into plexuses. Having thus shown how the bones are connected and Sut into motion, and from what sources their motion is erived, it may next be inquired how they and the other organs of the body grow and are nourished. This brings us to consider the blood and its ve els. The composition and properties of the blood, and the extraordinary changes which it suffers in its pa age through the pulmonary ve els, are elsewhere defined. (See Blood, and Respiration.) Without this exposure to the action of the air in the lungs, the blood is unfit for the support of life. We accordingly find that the heart is so constructed as to propel the blood which it receives through the structure of the lungs, and after it has there been aerated, to transmit it over the body: in fact, the heart is a hollow muscle: when it relaxes, its two principal cavities, or ventricles, are enlarged, and the blood flows in; when it contracts, they are diminished, and the blood is propelled into two large tubes or arteries, one leading to the lungs, and called the pulmonary artery, and the other to the system generally, and called the aorta: these arteries are not only elastic, but also muscular, so that they drive the blood onwards from the heart, its retrograde motion being effectually prevented by valves placed at their origin. The arteries are divided and subdivided into an infinite number of ramifications; and the branches from the same trunk are frequently observed to unite or anastomose in their course; so that when, bv any accident, some are obstructed, an adequate supply of blood may be kept up by the others. As, however, the blood caimot return to the heart by these ve els or arteries, we find that they inosculate, or communicate at their extremities 48 with another series of tubes or ve els, which are called veins. These are more numerous than the arteries, and generally accompany them in their course. They have a le muscular power; and as they are not a isted by the heart in propelling the blood, they open to it larger and larger channels as it advances, and are supplied with valves by which its reflux is prevented. This is, in fact, the circulation of the blood (first made out by Harvey, as before mentioned); the veins ultimately terminating in two large trunks which pour the blood into the right auricle of the heart; whence it is propelled into the right ventricle, from which arises the pulmonary artery, transmitting it through the lungs; from the lungs the blood (having been aerated) returns by the pulmonary vein into the left auricle of the heart, which contracting, propels it into the left ventricle, from which arises the aorta. Such, then is the extraordinary mechanism by which the circulation of the blood is effected; but it must not be supposed that the whole of the blood is thus directlv returned from the arterial into the venous system: a part of it is transmitted by minute arterial ramifications into the different structures and organs of which the body is composed, each of which is gifted with the power of a imilation, that is, of converting the blood, or a part of it, into a substance of its own kind. Some of these minute or capillary ve els also terminate upon the surface of the body, where they exhale perspirable matter; others, upon the membranes lining the cavities of the body, where they secrete the fluids which lubricate and moisten the interior surfaces; and others again go to the glands, — those peculiar organs or structures, which have not only the power of separating certain parts of the blood, but of converting it into new forms, which are called secretions, some of which are ejected, others retained, for the pu»poses of the animal economy. Thus, then, it appears that the blood nourishes and preserves the body and all its parts, and that it is continually tending to the renovation and reproduction of the different organs; but this very proce implies another, and no le extraordinary, function, which is performed by a distinct system; namely, that of absorption. There are, in short, a series of ve els which are continually carrying away the usele and worn-out materials; removing them in a state of solution; furnished, like the veins, with valves; terminating in a common trunk, called the thoracic duct; and pouring its contents into the veins, just before they enter the right auricle of the heart. It appears, therefore, that a continual system of deposition and removal is carrying on within tne living body; that the ramifications of the arterial system are constantly renovating the different organs, whilst the absorbents are as constantly removing the materials of which they consist. Nothing, therefore, is stationary or permanent; and as the blood, on the one hand, conveys the materials required, so, on the other, it receives those which are removed: and such as are usele , or would be hurtful if retained, are thrown off either by the intestines, the kidneys, the lungs, or the skin. It now only remains to show how this waste is compensated for, and by what means those materials which are thrown off in one form are replaced in another: this leads us to the functions of another branch of the aviimal machinery, called tlie organs of digestion; those organs, namely, by which the food is converted into blood. Different animals require different kinds and quantities of food; some living almost exclusively upon animal, others upon vegetable substances; hence their division into carnivorous and graminivorous tribes. Man partakes of both; and, accordingly, the structure of his digestive organs is intermediate between the comparative simplicity of the truly carnivorous, and the complexity of the graminivorous cla es. In all the higher orders of animals, however, the mechanism of digestion is of a complicated character. The first change which the food undergoes is in the mouth, where it is torn, ground, and moistened by machinery expre ly adapted to those operations. The teeth are admiral>ly contrived for this purpose; some of them cutting, and as it were mincing, others rubbing and grinding, whilst a fluid is supplied by the salivarj' glands so as to render the mixture of a proper consistency to be swallowed: this is effected by the organs of deglutition: the food is propelled from the mouth into the tube which conveys it to the stomach, and which is called the. oesophagus; and is, at the same time, prevented, by an extraordinary and complicated arrangement of the parts concerned, from pa ing in any other direction, and, more especially from cnteringthe trachea or air-pa age into the lungs. In the stomach the food is subjected to the secretions of that organ, called gastric juice, which is acid, and by which it is gradually converted into a greyish homogeneous semi-fluid substance, termed chyme; so that by the time the food htis reached the right end of tlie stomach, or the pylorus, its original characters are entirely changed; its separate materials are no longer discernible, and it has acquired distinct properties; it is, in short. Page 62