138 THE OSTEOLOGY AND MYOLOGY 



limbs, if some muscles are demonstrably homologous in the same sense that the femur and 

 humerus are, and many others afford nearly as unequivocal evidence to the same effect ; in 

 spite of the fact that some have not, as yet, been shown to be thus correlated. 



I do not venture to hope that all — indeed, I scarcely think that most — of the special 

 homologies I shall endeavor to point out will ultimately prove correct ; my range of ob- 

 servation has been too limited for this. But even faltering and devious advance, in such a 

 field as this, may be better than a stand-still ; something will be gained if any points are 

 firmly established ; and even errors themselves may serve some useful purpose when they 

 shall have been pointed out by better informed or more fortunate observers. 



Bearing in mind the position and relations of the skeleton already assumed, it will be 

 obvious that homologous muscles are to be sought for on opposite, which are homologous, 

 sides of the limbs ; that is, upon the front of the fore and back of the hind, or upon the 

 back of the fore and front of the hind, extremities, in the longitudinal direction ; and upon 

 the same side, that is, upon the outer aspect of the arm and outer aspect of the leg, or 

 upon the inner aspect of the arm and inner aspect of the leg, in the transverse direction. 

 There is no known departure from this rule. 



Position and relation are of first and last importance in the determination of muscular 

 homologies, and, in fact, afford the only sure guides. For muscles are so variously and 

 complexly modified, teleologically, for special purposes, that their function, or the end they 

 subserve in the movements of a limb, is of all things the least likely to afford correct indi- 

 cations. Even origin and insertion are not infallible indices ; for the same muscle varies in 

 these respects within certain limits. 



The terms " flexor" and "extensor," as ordinarily used, especially in anthropotomy, are 

 incorrect in their application to certain muscles of the forearm and hand. Primarily, 

 flexion is simply bending, and extension merely straightening, of the segment of a limb 

 with reference to the preceding one ; but in precise anatomical language these expressions 

 have come to have an arbitrary and special signification. Still recollecting the assumed 

 position of the limbs as above detailed, in which the axes of all the segments are more or 

 less oblique to each other, " flexion " is the increasing of this obliquity by lessening the 

 internal angles at the joints ; and " extension " is the lessening of this obliquity by increas- 

 ing the internal angles, till they equal 180°, and the several segments are straight with 

 reference to each other's axes, Avhen further extension at those joints where such motion is 

 possible reinduces obliquity, but in the contrary direction to that in which " flexion " 

 acted. Thus " flexor " muscles will invariably be found upon the back of the scapula, 

 forearm and hand, and upon the front of the pelvis, leg and foot ; upon the front of the 

 arm and the back of the thigh ; while conversely, " extensors ' ' lie upon the front of the 

 scapula, forearm and hand, and upon the back of the pelvis, leg and foot ; upon the back 

 of the arm and the front of the thigh. Applying this test, the so-called " flexors " upon the 

 front of the forearm and palmar aspect of the wrist are in reality extensors, and the so- 

 called " extensors " upon the back of the forearm and dorsum of the wrist are really flexors, 

 and will be so designated in succeeding paragraphs, although in what has gone before the 

 ordinary nomenclature has been observed. In reference to the leg and foot the usual inter- 

 pretiition of flexors and extensors is the correct one ; the same is the case regarding digits. 

 At the shoulder joint retroduction of the humerus is flexion, and conversely ; on the con- 

 trary, at the hip joint retroduction of the femur is extension, and conversely. All other 



