110 



M A M M A L I A. 



maintain the balance of the body in a great number 

 of attitudes. The metatarsal bones consist of five 

 elongated phalanges, like the metacarpal ones ; and 

 the phalanges of the toes are equal in number to 

 those in the fingers, only they are individually much 

 shorter, and their motions are far more limited ; and 

 though the great toe has no grasping motion against 

 the others, as the thumb has against the h'ngers, yet 

 it may be educated to far more activity, and may 

 acquire far more strength in its muscles than it has 

 in those who use their feet for no other purposes 

 than those of ordinary walking, and have them con- 

 fined and not unfrequently cramped in shoes from 

 their infancy. It has sometimes been said, and the 

 analogy at least is in favour of the truth of the saying, 

 that those who have been accustomed to go without 

 shoes in their earlier days are more vigorous in the 

 feet and retain the use of them longer than those 

 who wear shoes from the beginning. Over the joints 

 of the toes, and sometimes over those of the fingers, 

 and especially the last joint of the thumb, there are 

 sometimes little bones, formed not of diseased matter 

 but in a quite healthy state. These last are called 

 sesamoid bones, or milletseed bones ; and it is under- 

 stood that they act something in the same manner 

 as the knee-pan, in strengthening the joints and 

 increasing the facility of their motion. 



Such are the bones of the human subject, in their 

 arrangement, their application to each other, and their 

 principal uses. We have noticed the principal modes 

 in which bones are united by sutures, when it is not 

 intended in the economy of the system that the one 

 of them shall have a motion upon the other. We 

 have also mentioned some of the forms of those parts 

 of the moving bones which are applied to each other, 

 but it may not be amiss to recapitulate the principal 

 modes of those unions as they are usually stated in 

 the books. Of unions of bones which have motion 

 to a greater or less extent, there are two general 

 classes : those which are only partially moveable, or 

 partake something of the nature of sutures, and also 

 of the nature of joints ; and those which, whether 

 moveable in one direction or in several, or moveable 

 to a greater or less extent, have their motions free as 

 far as they go. The first species of union is called 

 amphiatkrosis, which means that they partake of one 

 or of both of the modes of separation or of junction ; 

 and the second is called dialhrosis, which means that 

 the bones are completely divided from each other, 

 unless in so far as they are bound together by liga- 

 ments. Cases of this last kind form the proper joints 

 of all mammalia ; and, therefore, they are of the 

 greatest consequence in studying the action of ani- 

 mals, though the others are scarcely less important 

 when we consider the adaptation of the animal struc- 

 ture to its own preservation, which is far from being 

 the least interesting, or the least instructive point of 

 view in which it can be regarded. 



The first modification of amphiathrosis is that in 

 which the bones are simply united by cartilaginous 

 matter: This mode of union is called synchondrosis, 

 because both bones stand in the same relation to the 

 uniting cartilaginous matter. Bones united in this 

 manner do not, strictly speaking, admit of motion ; but 

 they yield to pressure much more than bones which are 

 united by sutures. The different bones which com- 

 pose the pelvis are instances of this kind of union. 



There is a modification of the union by the mixture 

 of clastic ligament with the cartilaginous matter which 



admits of considerably more motion, and also is much 

 stronger, than the mere cartilage. The vertebrte ot 

 the spine, already alluded to, are examples of this 

 kind of union ; and we may remark, that whenever 

 flexure is given to a joint by muscular contraction, 

 the joint straightens again by the reaction of the 

 ligament whenever the muscular effort ceases. Liga- 

 ments more complete in their structure, and more 

 extended than these, are employed in many parts ol 

 the- structure of animals, as antagonists to muscles ; 

 and as the elasticity of the ligaments is a mere pro- 

 perty of matter, and not an effort of the living prin- 

 ciple, any repose, adhesion, or other result, which in 

 brought about by means of an elastic ligament, is rest 

 to the animal, and not fati<rue. A third form of par- 

 tial movement is that which gets the name of syn- 

 dermisis, which means that the bones are equally tied 

 together by ligaments surrounding them, but that 

 their surfaces are left free to slide upon each other, 

 This is the case with bones which merely modify 

 those motions which are conveyed across them by 

 tendons reaching from one bone to another; and the 

 most remarkable instances that the human body fur- 

 nishes of these are the numerous little bones in the 

 wrist and the instep, which modify but do not origi- 

 nate, by means of muscles attached to them, the 

 motions of the hands and feet. When one bone is 

 united to another by intervening muscles, and thus 

 imbedded in the flesh, the technical name given to il 

 is syssarcosis, which just means that the bone has its 

 lodgment in the flesh ; and of course it can movt 

 upon the other bones without the intervention of anj 

 thing else than those connecting muscles. The blade^ 

 bones are the most remarkable instances of this. 



In diathrosis, there are three original and simple 

 forms of articulation, with regard to the plane 01 

 planes in which the motions take place, and there is t 

 fourth one which is compound. The first is th 

 hinge-joint, which admits of motion in one plan* 

 only, that is, bending and extension of the limb 01 

 member, but not twisting or cross motion : the ar- 

 ticulation of the occipital bone upon the atlas, the 

 second and third joints of the fingers, and the joint! 

 of the elbow and knee, are chiefly of this kind, thougt 

 the application of the second bone turns these, ir 

 part at least, into compound motions. The seconc 

 form is rotatory motion, or motion in the same plane 

 all round and at right angles to the axis of the bones 

 forming the joint : the articulation of the atlas upor 

 the second or toothed vertebra of the the neck is the 

 most remarkable instance of this motion in the humar 

 body. A third form arises from the combination o 

 these two, which may be produced by the peculiai 

 form of the condyles and the cavities in which the) 

 act, or it may arise from a compound joint, where 

 two bones act upon one, and the one of them tends 

 to twist the other round, at the same time that the 

 joint is bent : the articulation of the lower jaw ir, 

 man and other mammalia, which have a grinding 

 motion of the jaw, is an example of compound 

 motion, produced by the form of the condyle 

 which allows one muscle to draw it laterally, while 

 another is drawing it upwards or downwards ; anc 

 all those joints in which a twist round and a flexure 

 can be produced at the same time by means of twc 

 bones, as in the elbow, and to a less extent in the 

 knee, are examples of the second form. 



The last form, and that which admits of the mosl 

 general motion, is the ball and socket joint, in which 



