IN THE LIMBS OF MAMMALIA. 71 



Here are enough examples to illustrate the general rule which may be stated in several 

 ways, according as we specially regard the muscles themselves, or the bones ; and these as 

 segments of insertion acted upon, or as segments of origin from which the muscles act. 



The muscles of the limbs form two groups, long and short ; the short muscles arising from 

 the first segment above that into which they are inserted, and the long from the second 

 segment above. A short muscle can act in but one way, but a long one has two actions : 

 one direct upon the segment into which it is inserted, the other indirect upon the segment 

 which intervenes between its origin and insertion. 



Each segment of a limb is flexed or extended by three muscles or groups of muscles- of 

 these, two are inserted into itself, and are therefore called direct muscles, flexors or extensors ; 

 one arising from the segment next above the segment to be moved, and hence called the 

 short direct, and the other arising from the second segment above, and hence called the long 

 direct ; the third muscle is termed the indirect flexor or extensor, and is the long direct exten- 

 sor ov flexor respectively of the next segment below, and between whose origin and inser- 

 tion intervenes the segment under consideration. 



Or it may be stated in yet a third way, vising the humerus as an example. Each seg- 

 ment not only gives insertion to the four muscles which act upon itself, short and long 

 extensors, deltoid and pectoral/s major, and flexors, teres major and latissimus dorsi, but 

 also affords origin to four more, of which two are the short flexor and extensor of the seg. 

 ment next below, brachialis anticus and humeral heads of triceps, and two, the long extensor 

 and flexor of the second segment below, extensor and flexor carpi radialis, which are also the 

 indirect extensor and flexor, respectively, of the first segment below. 



There are several advantages apparent in this arrangement of the muscles of the limbs, 

 and probably many more than have yet occurred to me. The most comprehensive is, that 

 while one half of the muscles, the short, are able to execute but one movement, since but 

 one joint intervenes between their origin and insertion, the other half may act in either of 

 two ways, since two joints intervene between their origin and their insertion. Take for 

 example the biceps humeri; its lower extremity is associated with the short flexor of the 

 fore-arm, brachialis anticus, which is thus its associate of insertion, while its upper extremity is 

 associated with the short extensor of the humerus, deltoid, which is thus its associate of 

 origin; in like manner its antagonist of origin is the short flexor of the humerus, teres major, 

 and its antagonist of insertion, the short extensor of the fore-arm, humeral heads of triceps. 

 Now when the elbow, its joint of insertion, is fixed by the simultaneous contraction of its 

 associate of insertion and its short antagonist of insertion, then the biceps can act only with 

 the deltoid, its associate of origin ; but when the shoulder, its joint of origin, is fixed by the 

 simultaneous contraction of its associate of origin and short antagonist of origin, then the 

 biceps acts with its associate of insertion to flex the fore-arm. 



Here is the great advantage in having both long and short muscles ; each long one may 

 act in two ways, and the short ones are then required to counteract one of the actions 

 while the other is performed. Moreover, it often, and in fact usually, happens, that two or 

 more segments of a limb are to be flexed or extended together, and this is provided for by 

 the same arrangement, with the additional fact that the short muscles are generally shorter 

 and thicker than the long. Suppose that two contiguous segments, as arm and fore-arm, 

 are to be extended at the same time. In this case neither of the short flexors, teres major 

 and brachialis anticus, can act at all, but the long flexor of the lower segment, biceps, is ena- 

 bled to act as the indirect extensor of the upper segment with its short associate of origin 

 deltoid, its flexor power being counteracted by its short antagonist of insertion, humeral 



