IN THE LIMBS OF MAMMALIA. 53 



In short, looking merely at man's body, beside being cosmopolitan and typically omniv- 

 orous, although it has a position and mode of locomotion peculiar to itself, and in which it 

 certainly is unrivalled, it is also endowed with the power to assume with grace almost every 

 conceivable attitude, and to employ at will the typical modes of locomotion of other Verte- 

 brates, such as swimming, crawling, leaping, and climbing ; and all these the human mind 

 has found means to outstrip in point of speed ; even the flying of the bird, though probably 

 it can never be equalled in rapidity, has been imitated by the aerial mode of locomotion 

 contrived by the same continent of man's essential superiority. 



But, leaving the mind wholly out of view, the human body is so constituted as not 

 only to best execute its own peculiar movements, but also to assume more readily than 

 the brutes some which are peculiar to other species. In other words, while endowed with 

 sufficient strength and firmness for all ordinary occasions, it has at the same time such 

 flexibility and independence of action as to be able to apply this strength in many and 

 very diverse ways. 



For clearer illustration let us contrast two extremes, the arm of man with the fore-leg of 

 the horse. The former can do nearly anything and everything except that of which alone 

 the latter is capable, namely, to support and propel the body on the earth ; yet in the two 

 limbs are the same joints, and, except in the hand, the same bones and muscles ; but in the 

 quadruped the latter are short and thick, and so disposed on the front and back of the limb 

 as to pull it with great force in those two directions, and in no other ; while in man they 

 are arranged evenly around the bony shafts, thus adding to the symmetry of the limb, as 

 well as increasing its mobility. 



But the most striking difference is, that, while the movements of each segment of the 

 human arm are, if necessary, entirely independent of those of the other segments, in the horse 

 they are much less so, and flexion or extension at the elbow causes a mechanical movement 

 at the wrist, and vice versa : the independence of the movements of these two joints seems 

 to correspond with the degree of development of the humeral condyles ; these processes, 

 when they exist, are situated just above the insertions of the external and internal lateral 

 ligaments of the elbow, at which two points there is of course no motion ; the condyles lie 

 a little above and therefore change position, though very slightly, during movement at the 

 joint. The extensor muscles of the wrist and fingers arise from the external condyle, the 

 flexors from the internal, both processes being very prominent in man ; but it is evident 

 that when the condyles are very small or absent, the origins of the muscles must . in like 

 degree reach above or below the centres of motion, and thus, with the parts into which they 

 are inserted, be more or less affected by any movements at the joint. Now in the horse 

 the condyles are almost wholly wanting, the flexors of the hand arising below the centre 

 of motion on the inner side of the humerus, and the extensors above the corresponding 

 point on the outer side ; when, therefore, the hand is flexed, the humerus and fore-arm 

 are also flexed at the elbow, and when the hand is extended, these other segments are also 

 extended. 



Hold the fore-leg of a horse horizontally by the part between the elbow and wrist, and 

 flex the hand ; the limb bends at the elbow also. Now if you rest the limb in its natural 

 position upon the earth, the obliquity in the direction of the hoof tends to extend the hand 

 at the wrist, and thus to straighten the limb at the elbow, so that the heavier you press 

 upon the top of the humerus, the firmer the limb becomes. It is evident that this would 

 be a mechanical aid in sustaining the weight of the animal, but I have had no opportunity 



MEMOIRS BUST. SOC. NAT. HIST. Vol. I. 14 



