74S 



MECHANICS, (ANIMAL.) 



socket of the arm, slides about itself upon the convex 

 t-xtfiior of the chest, having its motion limited only 

 by a connexion, through the collar bone, or clavicle, 

 with the sternum. 



The scapula, or blade-lone, is extraordinary as an 

 illustration of the mechanical rules for combining 

 lightness with strength. It has the strength of the 

 arch, from being a little concave, and its substance is 

 chiefly collected in its borders and spines, with thin 

 plates between, as the strength of a wheel is col- 

 lected in its rim, and spokes, and nave. 



The bones of the arms, considered as levers, have 

 the muscles which move them attached very near to 

 the fulcra, and very obliquely ; so that, from working 

 through a short distance comparatively with the re- 

 sistances overcome at the extremities, the muscles 

 require to be of great strength. It has been calcu- 

 lated that the muscles of the shoulder-joint, in the 

 exertion of lifting a man upon the hand, pull with a 

 force of two thousand pounds. 



The os humeri, or bone of the upper arm, is not 

 perfectly cylindrical; but, like most of the other 

 bones which are called cylindrical, it has ridges to give 

 strength. 



The elbow-joint is a correct hinge, and so strong- 

 ly secured, that it is rarely dislocated without frac- 

 ture. 



The fore-arm consists of two bones, with a strong 

 membrane between them. Its great breadth, 

 from this structure, affords abundant space for the 

 origin of the many muscles that go to move Uie 

 hand and fingers ; and the very peculiar mode of 

 connexion of the two bones, give man that most 

 useful faculty of turning the hand round, into 

 what are called the positions of pronation and supi- 

 nation, exemplified in the action of twisting, or of 

 turning a gimlet. 



The wrist. The many small bones forming this, 

 have a signal effect of deadening, in regard to the 

 parts above, the shocks or blows which the hand 

 receives. 



The annular ligament is a strong band passing 

 round the joint, and keeping all- the tendons which 

 pass from the muscles above to tlie fingers, close to 

 the joint. It answers the purpose of so many fixed 

 pulleys for directing the tendons : without it, they 

 would all, on action, start out like bow-strings, pro- 

 ducing deformity and weakness. 



The human hand is so admirable, from its numer- 

 ous mechanical and sensitive capabilities, that an 

 opinion at one time commonly prevailed, that man's 

 superior reason depended on his possessing such an 

 instructor and such a servant. Now, although rea- 

 son, with hoofs, instead of fingers, could never have 

 raised man much above the brutes, and probably 

 could not have secured the continued existence of the 

 species, still the hand is no more than a fit instrument 

 of the godlike mind which directs it. 



The pelvis, or strong irregular ring of bone, on the 

 upper edge of which the spine rests, and from the 

 sides of which the legs spring, forms the centre of 

 the skeleton. A broad bone was wanted here to 

 connect the central column of the spine with the lat- 

 eral columns of the legs ; and a circle was the 

 lightest and strongest. If we attempt still further 

 to conceive how the circle could be modified to fit it 

 for the spine to rest on, for the thighs to roll in, for 

 muscles to hold by, both above and below, for the 

 person to sit on, we shall find, on inspection, that all 

 our anticipations are realized in the most perfect 

 manner. In the pelvis, too, we have the thyroid 

 hole and ischiatic notches, furnishing subordinate 

 instances of contrivance to save material and weight: 

 they are merely deficiencies of bone where solidity 

 .XHild not have given additional strength. The broad 



ring of the pelvis protects most securely the important 

 organs placed within it. 



The /tip-joint exhibits the perfection of the ball 

 and socket articulation. It allows the foot to move 

 round in a circle, as well as to have the great range 

 of backward and forward motion exhibited in the 

 action of walking. When we see the elastic, tough, 

 smooth cartilage which lines the deep socket of this 

 joint, and the similar glistening covering of the ball 

 or head of the thigh bone, and the lubricating syno- 

 via poured into the cavity by appropriate secretaries, 

 and the strong ligaments giving strength all around, 

 we feel how far the most perfect of man's works falls 

 short of the mechanism displayed by nature. 



The thigh-bone is remarkable for its projections 

 called trochanters, to which the moving muscles are 

 fixed, and which lengthen considerably the lever by 

 which the muscles work. The shaft of the bone is 

 not straight, but has a considerable forward curva- 

 ture. Short-sightedness might suppose this a weak- 

 ness, because the bone is a pillar supporting a weight; 

 but the bend gives it, in reality, the strength of the 

 arch, to bear the action of the mass of muscle 

 called vastus, which lies and swells upon its fore 

 part. 



The knee is a hinge joint of complicated structure; 

 and it claims the most attentive study of the surgeon. 

 The rubbing parts are flat and shallow, and therefore 

 the joint has little strength from form ; but it derives 

 security from the numerous and singularly strong 

 ligaments which surround it. The ligaments in the 

 inside of the knees resemble in two circumstances, 

 the annular ligaments of joints, namely, in having" a 

 constant and great strain to bear, and yet in becom- 

 ing stronger always as the strain increases. The line 

 of the leg, even in the most perfect shapes, bends 

 inward a little at the knee, requiring the support of 

 the ligaments, and in many persons, it bends very 

 much ; but the inclination does not increase with 

 age. The legs of many weakly in-kneed children 

 become straight by exercise alone. This inclination at 

 the middle joint of the leg, by throwing a certain 

 strain on the ligaments, gives an increase of elasti- 

 city to the limb, in the actions of jumping, running, &c. 

 In the knee, there is a singular provision of loose 

 cartilages, which have been called friction carti- 

 lages, from a supposed relation in use to friction 

 wheels ; but their real effect seems to be to accom- 

 modate, in the different positions of the joint, the 

 surfaces of the rubbing bones to each other. 



The great muscles on the fore part of the thigh 

 are contracted into a tendon a little above the knee, 

 and have to pass over, and, in front of the knee, to 

 reach the top of the leg, where their attachment is. 

 The tendon, in passing over the joint, becomes bony, 

 and forms the patella, or knee-pan, often called the 

 pulley of the knee. This peculiarity enables the 

 muscles to act more advantageously, by increasing 

 the distance of the rope from the centre of motion. 

 The patella is, moreover, a sort of shield or protec- 

 tion to the fore part of this important joint. The 

 leg below the knee, like the fore-arm already 

 described, has two bones. They offer spacious 

 surface of origin for the numerous muscles required 

 for the feet, and they form a compound pillar of 

 greater strength than the same quantity of bone as 

 one shaft would have had. The individual bones 

 also are angular instead of round, hence deriving 

 greater power to resist blows, &c. 



The ankle-joint is a perfect hinge of great strength. 

 There is in front of it an annular ligament, by which 

 the greater part of the tendons, passing downwards 

 to the foot and toes, are kept in their places. 

 One of these tendons passes tinder the bony pro- 

 jection of the inner ankle, in a smooth -appropriate 



