The substances which connect the bo dies of the vertebra 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 for wards, the compressible cartilage, yield ing to the force, brings the surfaces of the adjoining vertebra 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.cornpressing force ceases.
In order still further to increase the strength of the compagcs, and to add a greater security against lusation, the ver tebrz 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 acci dentally 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 officesof life, are performed with safety, but that it is an accident hardly ever heard of, that even the ges ticulations of a harlequin distort his spine.
The ribs are articulated by their pos terior extremities to the bodies and to the transverse processes of the vertebra, 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.
faints ti the upper extremity. —The clavi cle is articulated to the sternum at one end, and to the scapula at the other.
The shoulder is formed by a round head of the humerus, which plays in a cup of the scapula ; and the ends of the bones arc inclosed by a thick and strong ligamentous membrane, called the orbi cular 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 restrain ed by strong ligaments placed at the sides of the joint, and the fore-arm can there fore be moved only forwards and back wards. 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 suvination, are per formed 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 articu lated by ginglymi.
The bones of the pelvis are inseparably connected by adhering cartilaginous sur faces 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 guard ed and strengthened. There is a very hirge rounded head of the thigh received into a deep cup of the os innominatum. I lere it can revolve freely, and is prevent ed 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 impor tant 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 obsta cle 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 gin glymus, and its motions are accordingly restrained by two strong lateral ligaments, and it is secured still further by two im mense ligamentous ropes withm the ca vity of the joint, called the crucial liga tnents.
The ankle is a ginglymoid joilit, formed by the tibia and fibula, togcther with the a_strag-alus. This joint, which is an im portant 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.