« PreviousContinue »
of which do not reach so far; are called the false ribs. The sternum is a broad and flat bone, placed in the front of the chest. It consists of two pieces of bone, and of a cartilage called the ensiform. 'l'he clavicles are articulated towards its upper parts, and the cartilages of the ribs are joined to its sides. The pelvis is formed by the two ossa innominata, or haunch bones, the sacrum, and os coccygis. The former are very large and flat bones, expanded into a broad surface above for the support of the abdominal viscera, and the attachment of the abdominal muscles, and furnished with large tuberosities below, for the support of the body in the sitting position. Each os innominatum is divided into the ilium, ischium, and pubes. It is firmly joined to the sacrum behind, and to the opposite bone in front by the symphisis pubis. The conjoined portions form an arch, called the arch of the pubes. The cavity of the pelvis is much larger in the female than in the male, as it holds the uterus and vagina in addition to what it contains in the male, and as the foetus passes through it in parturition. The bones of the upper extremity are distributed into those of the shoulder, arm, fore-arm, and hand. The shoulder contains two; the scapula and clavicle. The former is situated at the upper and outer part of the chest, and is joined to the end of the clavicle. The humerus is a long and nearly cylindrical bone, joined by a round head to the scapula above, and articulated with the radius and ulna below. The fore-arm has two bones; the ulna, which is joined by a hinge or ginglymus to the humerus; and the radius, which has a cavity playing upon a rounded head of that bone. The prominent extremity of the ulna, which forms the elbow, is called the olecranon. The hand is divided into the carpus, or wrist, the metacarpus, and the fingers and thumb. The carpus contains eight bones, disposed in two phalanges, of which the first forms, with the radius, the joint of the wrist, and the second is articulated to the metacarpus. The bones of the first phalanx are the os naviculare, lunatum, cuneiforme, and orbiculare: those of the second, os trapezium, trapezioides, capitatum, and unciforme. The metacarpus has five bones, and each of the fingers three; the thumb only two. In the lower extremity we have the femur, the largest of the cylindrical bones in the body. This has a round head, contained
in a socket of the os innominatum: the great trochanter forms a conspicuous process at the upper and outer part of the bone. Below it has two condyles, which form part, of the knee. The leg has two bones; the tibia and fibula. A large flat portion of the former, covered only by skin, is called the shin. The foot is composed of the tarsus, metatarsus, and toes. The tarsus has seven bones:1. Astragalus, composing the ankle, with the lower portion of the tibia and fibula. 2. Os calcis. 3. Os naviculare. 4. Os cuboides. 5, 6, 7. Ossa cuneiformia. The metatarsal bones are five in Irumber, and the bones of each toe are three, except the great toe, which has only two.
SYN Des MologY, on DocTRINE OF THE JOINTs.
Construction of a joint. —The opposed surfaces of bones, which form joints, are covered by a thin crust of cartilage, most exquisitely smooth and polished. Hence they move on each other in whatever direction their structure admits, without any hindrance from friction. They are tied to. gether by strong and unyielding cords, resembling tendons, and known by the name of ligaments. These keep the surfaces of the bones together, and restrict their notions to certain directions. In order still further to promote the facility of motion, and to obviate every possibility of friction, the cartilaginous surfaces are smeared with an unctuous fluid, called synovia,which makes them perfectly slippery. This fluid is confined to the surface of the joint by means of a thin and delicate membrane, called the capsular ligament, which envelopes the joint. It is secreted from portions of a fatty substance, called the synovial glands. The ligaments are usually situated on the outside of the capsula; but in many instances they are contained in the cavity of the joint, passing from the centre of one bone to another. These are called interarticular ligaments.
Particular joints.-Joint of the lower jaw. This is formed between the condyle of the jaw, and a hollow in the temporal bone. It contains a moveable cartilage, which ren-ders the articulation more secure, when the jaw is brought forwards on the bone under certain circumstances.
The connection of the head to the vertebrae is effected by means of two prominences of the occiput, which are received into corresponding cavities of the atlas. By this joint the modding motions of the head are performed. But the atlas itself turns hori
zontally round the tooth-like process of the vertebra dentata, and as the head is closely connected to the atlas, it is carried round at the same time. Therefore the lateral or rotatory motions of the head are performed by a different joint from that which performs the nodding motions. Neither of these articulations admits of very extensive motion; but the deficiency is compensated by the mobility of the vertebrae, which enable us to carry the head freely in any direction we may wish. The head rests nearly in equilibrio on the spinal column; yet, if left to itself, it would fall forwards, as the joint is not precisely in the centre of the basis cranii. To counteract this tendency there is a ligamentous substance extended from the spinous processes of the cervical vertebrae to the occiput, and called the ligamentum nuchae. In quadrupeds this can be best seen, as the weight of the head is there supported to a much greater disadvantage. The muscles also contribute to keep the head upright; and hence, when a man drops asleep sitting, the relaxation of the extensor muscles causes the head to nod forwards. Joints of the spine.—The spine, or backbone, is a chain of joints of very wonderful construction. Various, difficult, and almost inconsistent offices were to be executed by the same instrument. It was to be firm, yet flexible; firm, to support the erect position of the body; flexible, to allow of the bending of the trunk in all degrees of curwature. It was further also to become a pipe or conduit for the safe conveyance of a most important part of the animal frame, the spinal marrow; a substance, not only of the first necessity to action, if not to life, but of a nature so delicate and tender, so susceptible, and so impatient of injury, as that any unusual pressure upon it, or any considerable obstruction of its course, is followed by paralysis or death. It was also to afford a fulcrum, stay, or basis for the insertion of the muscles which are spread over the trunk of the body, in which trunk there are not, as in the limbs, cylindrical bones to which they can be fastened; and hikewise, which is a similar use, to furnish a support for the ends of the ribs to rest upon. The breadth of the bases, upon which the parts severally rest, and the eloseness of the junction, give to the chain its firmness and
stability; the number of parts, and conse. .
quent frequency of joints, its flexibility; which flexibility, we may also observe, vaties in different parts of the chain; is least
in the back, where strength more than flexure is wanted; greater in the loins, which it was necessary should be more supple than the back; and greatest of all in the neck, for the free motion of the head. Then, secondly, in order to afford a passage for the descent of the medullary substance, each of these bones is bored through in the middle in such a manner, as that, when put together, the hole in one bone falls into a line and corresponds with the holes in the two bones contiguous to it; by which means the perforated pieces, when joined, form an entire, close, uninterrupted channel. But, as a settled posture is inconsistent with its use, a great difficulty still remained, which was to prevent the vertebrae from shifting upon one another, so as to break the line of the canal as often as the body moves or twists, or the joints gaping externally whenever the body is bent forwards, and the spine thereupon made to take the form of a bow. These dangers, which are mechanical, are mechanically provided against. The vertebrae, by means of their processes and projections, and of the articulations which some of these form with one another at their extremities, are so locked in and confined, as to maintain in what are called the bodies or broad surfaces of the bones the relative position nearly unaltered; and to throw the change and the pressure produced by flexion almost entirely upon the intervening cartilages, the springiness and yielding nature of whose substance admits of all the motion which is necessary to be performed upon them, without any chasm being produced by a separation of the parts. I say of all the motion which is necessary; for, although we bend our backs to every degree almost of inclination, the motion of each vertebra is very small: sich is the advantage which we receive from the chain being composed of so many links. Had it been composed of three or four bones only, in bending the body the spinal marrow must have been bruised at every angle. The substances which connect the bodies of the vertebrae to each other, called the intervertebral cartilages, are thick, firm, and elastic. They are similar in shape, and nearly so in size to the bones which they join. They are thicker before than behind, so that, when we stoop forwards, the compressible cartilage, yielding to the force, brings the surfaces of the adjoining vertebrae nearer to a state of parallelism than they were before, instead of increasing the inclination of their planes, which must have occasioned a fissure or opening between them:
and their elasticity restores the body to its former state, when the compressing force ceases. In order still further to increase the strength of the compages, and to add a greater security against luxation, the vertebrae are articulated to each other by means of the processes before mentioned. And these processes so lock in with and overwrap one another as to secure the body of the vertebra, not only from accidentally slipping, but even from being pushed out of its place by any violence short of that which would break the bone. The roots of the spinous processes are also joined to each other by very strong and highly elastic ligamentous substances, which will tend powerfully to restore the column after it has been bent forwards. The general result is, that not only the motions of the human body necessary for the ordinary offices of life are performed with safety, but that it is an accident hardly ever heard of that even the gesticulations of a harlequin distort his spine. * The ribs are articulated by their posterior extremities to the bodies and to the transverse processes of the vertebrae, and the true ribs are also joined by means of their cartilages to the sternum. Two great advantages are derived from the ribs having this cartilaginous portion. The effect of blows, or of any accidental violence, is eluded by the flexibility which they thus obtain; and the elastic power of the cartilages restores the ribs to their former position, after they have been raised by the intercostal muscles in breathing. Joints of the upper extremity.—The clavicle is articulated to the sternum at one end, and to the scapula at the other. The slioulder is formed by a round head of the humerus, which plays in a cup of the scapula; and the ends of the bones are inclosed by a thick and strong ligamentous membrane, called the orbicular ligament. There is here, therefore, every latitude of motion allowed. In the elbow, on the contrary, the joint is a mere hinge: lateral motion is restrained by strong ligaments placed at the sides of the joint, and the fore-arm can therefore be moved only forwards and backwards. This joint is formed between the ulna and the humerus. The wrist is formed by the junction of the radius with the first phalanx of carpal bones. Its motion is very little more than that of a ginglymus. The rotation of the
hand and wrist, or what anatomists call the pronation and supination, are performed by the radius revolving round the ulna, and carrying the hand with it. In this case the elbow joint is fixed; neither does the joint of the wrist move; but the radius moves freely round the ulna, and the hand is included in the motion. The pronation and supination of the hand are well exemplified in the use of the broad-sword, and in cudgel-playing. The carpal and metacarpal bones are united by joints and ligaments, but have no obvious motion on each other. The phalanges of the fingers are also articulated by ginglymi. The bones of the pelvis are inseparably connected by adhering cartilaginous surfaces and immense ligaments. Such is the strength of this union, that it will yield to no force but one that would destroy and crush the whole fabric. Joints of the lower extremity—In the hip, which supports the whole body, and which is the centre of motion of the whole in moving from place to place, we find an apparatus admitting of extensive motion, but at the same time most carefully guarded and strengthened. There is a very large rounded head of the thigh received into a deep cup of the os innominatum. Here it can revolve freely, and is prevented from escaping by thick and strong rising edges, that guard the brim of the cavity. From these edges there springs a very tough and stout orbicular ligament, which is firmly stretched over the head of the bone, and implanted into a contracted part called the neck. In order to provide still further for the security of so important a joint as the hip, there is a short, strong ligament arising from the head of the ball, and implanted in the bottom of the cup. This affords a very great obstacle to any force tending to displace the bone; but at the same time lies in the bottom of the cavity, so as not to interfere with any of the ordinary motions. The knee-joint is formed by three bones: the head of the tibia, the condyles of the femur, and the patella. It is a ginglymus, and its motions are accordingly restrained by two strong lateral ligaments, and it is secured still further by two immense ligamentous ropes within the cavity of ‘the joint, called the crucial ligaments. The ankle is a ginglymoid joint, formed by the tibia and fibula, together with the astragalus. This joint, which is an important one, as bearing the weight of the whole body, is strengthened at its sides by two bony processes, called the internal and external malleoli or ankles. The bones of the tarsus, metatarsus, and toes, are articulated like those of the hand.
Muscles consist of bundles of red fibres; but the colour is not essential, since it can be removed by repeated washings and maceration. The threads composing a muscle are enveloped by cellular substance, which connects it to the surrounding parts. Each bundle consists of numerous fibres, so small that our instruments of research cannot arrive at the ultimate or original fibre: hence any perceivable fibre, however small, is formed by the juxta-position of numerous fibrillae; and, as we employ magnifying instruments of greater power, a fibre, which before seemed simple, resolves itself into a congeries of still more minute threads. We pass over in silence the dreams of various investigators who have busied themselves in looking for the ultimate muscular fibre; these researches do not assist us in explaining the phenomena of muscular action. The cohesion of the constituent particles of the moving fibre is maintained by the vital power: hence a dead muscle will be torn by a weight of a few ounces, which in the living body would have supported many pounds. The muscular fibre receives a copious supply of vessels and nerves. Tendons are formed by an assemblage of longitudinal parallel fibres. They are extremely dense and tough, of a splendid white colour, which is beautifully contrasted with the florid red of a healthy muscle. The muscular fibres terminate in these bodies, and they are connected to the bones. They possess no apparent nerves, and very few and small blood-vessels. There is always an exact relation between the joint and the muscles that move it. Whatever motion the joint, by its mechanical construction, is capable of performing, that motion the annexed muscles by their position are capable of producing. For example, if there be, as at the knee and elbow, a hinge joint, capable of motion only in the same plane, the muscles and tendons are placed in directions parallel to the bone, so as by their construction to produce that motion and no other. If these joints were capable of a freer motion, there are no muscles to produce it. Whereas, at the shoulder and the hip, where the ball and
socket joint allows by its construction a retatory or sweeping motion, tendons are placed in such a position, and pull in such a direction, as to produce the motion of which the joint admits. In the head and hand there is a specific mechanism in the bones for rotatory motion; and there is accordingly in the oblique direction of the muscles belonging to them a specific provision for putting this mechanism of the bones into action. The oblique muscles would have been inefficient without that particular articulation, and that particular articulation would have been useless without the muscles. As the muscles act only by contraction, it is evident that the reciprocal energetic motion of the limbs, or their motion with force in opposite directions, can only be produced by the instrumentality of opposite or antagonist muscles, of flexors and extensors answering to each other. For instance, the biceps and brachialis internus, placed in the front of the arm, by their contraction bend the elbow, and with such degree of force as the case requires, or the strength admits of. The relaxation of these muscles after the ef. fort would merely let the fore-arm drop down: for the back stroke therefore, and that the arm may not only bend at the elbow, but also extend and straighten itself with force, other muscles, as the triceps and anconeus, placed on the hinder part of the arm, fetch back the fore-arm into a straight line with the humerus with no less force than that with which it was bent out of it. It is evident therefore that the animal functions require that particular disposition of the muscles, which we call antagonist muscles. It often happens that the action of muscles is wanted, where their situation would be inconvenient. In which case, the body of the muscle is placed in some commodious position at a distance, and it communicates with the point of action by slender tendons. If the muscles, which move the fingers, had been placed in the palm or back of the hand, they would have swelled that part to an awkward and clumsy thickness. The beauty, the proportions of the part wonld have been destroyed. They are therefore disposed in the arm, and even up to the elbow, and act by long tendons strapped down at the wrist, and passing under the ligament to the fingers, and to the joints of the fingers, which they are severally to move. In the same manner the muscles which move the toes and many of the joints of the foot, are gracefully disposed in the calf of the leg, instead of forming an unwieldy tumefaction in the foot itself.
The great mechanical variety in the fignre of the muscles may be thus stated. It appears to be a fixed law, that the contraction of a muscle shall be towards its centre. Therefore the subject for mechanism on each occasion is, so to modify the figure, and adjust the position of the muscle, as to produce the motion 1equired, agreeably with this law. This can only be done by giving to different muscles a diversity of configuration, suited to their several offices, and to their situation with respect to the work, which they have to perform. On which account we find them under a multiplicity of forms and attitudes; sometimes with double, sometimes with treble tendons, sometimes with none: sometimes one tendon to several muscles, at other times one muscle to several tendons. The shape of the organ is susceptible of an incalculable variety, whilst the original property of the muscle, the law and line of its contraction, remains the same, and is simple. Herein the muscular system may be said to bear a perfect resemblance to our works of art. An artist does not alter the native quality of his materials, or their laws of action. He takes these as he finds them. His skill and ingenuity are employed in turning them, such as they are, to his account, by giving to the parts of his machine a form and relation, in which these unalterable properties may operate to the production of the effects intended.
The muscular system would afford us numerous examples of what may be called mechanical structure: i.e. of such contrivances employed to attain certain objects, as a human artist would adopt on similar occasions. One of the muscles of the eyeball presents us with a very perfect pulley; by means of which the globe of the eye is moved in a direction exactly contrary to the original application of the force. This muscle, which is called the trochlearis, arises from the very back part of the orbit; it has a long and slender tendon running through a pulley in the inner part of the front margin of the orbit, and then going back to be fixed in the hind portion of the eye-ball. Thus it draws the globe obliquely upwards and forwards, although the line of the contraction of the muscle is directly backward.
In the toes and fingers, the long tendon, which bends the first joint, passes through the short tendon, which bends the second joint.
The foot is placed at a considerable angle with the leg. It is manifest, therefore, that flexible strings, passing along the interior of the angle, if left to themselves, would, when stretched, start from it. The obvious preventive is to tie them down, and this is done in fact. Across the instep, or rather just above it, the anatomist finds a strong ligament, under which the tendons pass to the foot. The effect of the ligament, as a bandage, can be made evident to the senses; for if it be cut, the tendons start up. The simplicity, yet the clearness of this contrivance, its exact resemblance to established resources of art, place it among the most indubitable manifestations of design, with which we are acquainted. The number of the muscles of the human body is so great, and the circumstances, which demand attention in every muscle are likewise so numerous, that a particular description of each would extend this article beyond its prescribed limits. We shall therefore merely give a catalogue of the muscles; which together with the references to the annexed plates, will give the reader a sufficiently clear motion of the subject. Muscles of the scalp.–1. Fronto-occipitalis, or epicranius. Muscles of the ear.—1. Attollens auriculam; 2. anterior auris; 3, 4. retrahentes auriculam; 5. major helicis; 6. minor helicis; 7. tragicus; 8. antitragicus; 9. transversus auriculae; 10. laxator tympani major; 11. laxator tympani minor; 12, tensor tympani; 13, stapedeus. Muscles of the eye.—1. Orbicularis palpebrarum; 2.'corrugator supercilii; 3. levator palpebrae superioris; 4. attollens oculi; 5. abductor oculi; 6. depressor oculi; 7. adductor oculi, these are also called recti: viz. rectus superior, externus, inferior, and internus;8. obliquussuperior oculi, or trochlearis; 9. obliquus inferior oculi. Muscles of the nose.—1. Compressor narium; 2. levator labii superioris et alae nasi; 3.nasalis labiisuperioris; 4.depressor alaenasi. Muscles of the lips.-1. Levator labii superioris; 2. zygomaticus major ; 3. zygomaticus minor; 4. levator anguli oris; 5. depressor anguli oris; 6. depressor labii inferioris; 7. buccinator; 8. orbicularis oris; 9. anomalus maxillae superioris; 10. levator menti. Lower jaw.—1. Biventer maxillae or digastricus; 2. masseter; 3. temporalis; 4. pterygoideus externus; 5. pterygoideus insternus.