IN THE LIMBS OF MAMMALIA. 27 



heads of triceps, which, being more powerful, continues to act as an extensor, beside leaving 

 the long extensor, scapular head of triceps entirely free ; the action of the latter, as an indirect 

 flexor of the humerus, being in like manner more than counteracted by the short extensor 

 of the same, deltoid. But if the same two segments are to be simultaneously flexed, then 

 the short extensors do not act at all, while the extensor power of the long direct flexor, 

 biceps, is overbalanced by its short antagonist of origin, teres major, and that of the long 

 direct extensor, scapular head of triceps, by its short antagonist of insertion, brachialis anticus, 

 leaving the two long flexors, latissimus dorsi and biceps, free as flexors of the two segments ; 

 moreover, the long rotators of the fore-arm, supinator longus and pronator teres, with the long 

 extensor of the wrist, extensor carpi radialis, and here by exception, even the long flexor of 

 the third segment below, flexor communis digitorum sublimis, now act as indirect flexors of the 

 fore-arm, their direct actions being, if necessary, prevented by their short antagonists, or in 

 the case of the rotators by each other. 



Again, as has perhaps been already inferred, the three muscles acting upon any one seg- 

 ment, are, potentially or morphologically, of the three degrees of length in direct ratio, but of 

 thickness in inverse ratio ; and since the power of a muscle is in proportion to its thickness, 

 and the distance through which it can contract, according to the length of its fibres, it fol- 

 lows that a movement is most rapid and forcible at the beginning, when all three muscles 

 act together, and least so at its close when the longer and weaker muscles act alone ; and 

 this agrees with the observations on the variation in the force of contraction of a single 

 muscle, which is found to be most forcible at the beginning, least so at the end. 



I have now stated the general law and the general advantages gained thereby as illus- 

 trated in those regions where the muscles are most familiar and where the law is quite 

 closely adhered to, yet even here were some departures from it ; and in the remaining por- 

 tions of the limbs will be found even more variations, according to the number and kind of 

 movements required at the several joints. 



There is considerable doubt with regard to the muscles acting upon the scapula, and for 

 two principal reasons, one teleological and the other morphological ; for, beside the compli- 

 cation necessary for the very free movement upon the walls of the thorax, of a part which 

 in many mammalia has no bony connection therewith, there is so much difficulty in compre- 

 hending the true relations of some muscles arising from the head and cervical vertebrae, 

 and inserted into the scapula and clavicle, as to afford additional, though for the present 

 vague, evidence concerning that remarkable change, by which the scapular arch has been 

 displaced backward from the occipital cranial vertebra, of which it is in most fishes actu- 

 ally, and in all vertebrates morphologically, the pair of ribs. 



The scapula slides upon the sides of the thorax, and is separated from it chiefly by mus- 

 cles, having in most mammalia no bony connection with the rest of the skeleton ; and the 

 muscles which act directly upon it, are, like those of the face, inserted at once into the part 

 to be moved, without the additional mechanical complication of levers. It may be elevated 

 and depressed, drawn forward toward the sternum, or backward toward the vertebral 

 column. Two direct muscles draw the scapula forward; one, the pectoralis minor, inserted 

 into the coracoid process, the other, the serratus magnus, attached along the posterior bor- 

 der, both arising from the ribs ; two direct muscles also draw it backward, the trapezius 

 outside, inserted upon the clavicle and spine of the scapula, and the rhomboideus inside, in- 

 serted into the posterior border, and both these arising from the vertebral column. Here 

 then, as usual, are two flexors and two extensors, but as there is no part beyond the verte- 



