IN THE LIMBS OF MAMMALIA. 73 



bral column or sternum on the middle line, all four are Ideologically short muscles, since but 

 one joint intervenes between their origins and insertions ; it seems likely, however, that in a 

 morphological point of view, the outer ones, trapezius and pectoralis minor, are the long mus- 

 cles. The scapula is elevated and depressed directly by the oblique descending and ascend- 

 ing fibres of the muscles already described or indirectly by the long muscles of the humerus 

 pectoralis major and latissiinus dorsi. The true affinities of the cleido-mastoid and levator clavic- 

 ulce have not yet been determined ; the omo-hyoid is probably the representative of an 

 intercostal muscle connecting the ribs of the occipital and parietal vertebrae ; the leva- 

 tor anguli scapula is commonly regarded as a distinct muscle, but from some facts in its 

 comparative anatomy, it appears to me rather as a dismemberment of the serratus magmts. 

 All this is rather unsatisfactory, and will, I think, continue so till embryology and com- 

 parative anatomy have demonstrated the true relations of these muscles, and the extent to 

 which they vary from the common type of the intercostales ; for it must be remembered, 

 that the scapula and its posterior representative, the ilium, are not, properly speaking, seg- 

 ments of the limbs, but the dorsal moities or pleur apophyses of modified ribs, at the junction 

 of which with the ventral moities or hceniapophyses, are given off diverging appendages in 

 the shape of the anterior and posterior extremities. 



The deltoid is the short extensor of the humerus, arising, as it should, from the scapula ; 

 either the long extensor is wanting, or, as is more probable, it is represented by the pecto- 

 ralis major, or one of the several subdivisions of the latter, existing in quadrupeds, where 

 the clavicle is deficient. The long and short flexors of the arm, latissiinus dorsi and teres 

 major, have been already noticed. With regard to the supra and infra spinati, the subscapw- 

 laris and teres minor, I am not yet decided; in man they are chiefly rotators of the limb at 

 the shoulder, but in quadrupeds where the humerus is simply flexed or extended, the 

 three former have more or less power of extension, while the teres minor is a flexor. The 

 muscles acting upon the fore-arm have been already described ; the dorso-epitrochlien of 

 Duvernoy (" Des caracteres anatomiques des grands singes pseudo-anthropomorphes," 

 Archives du Museum, vol. viii.) which exists in the quadrumana and most of the mamma- 

 lia, has nearly the same relations as the scapular head of the biceps, being, when inserted 

 upon the olecranon, a direct extensor of the fore-arm as well as an indirect flexor of the 

 humerus, which alone it flexes directly when attached to the internal condyle. 



The relations of the biceps in the higher mammalia are somewhat complicated by the pro- 

 visions for rotation between the two bones of the fore-arm, converting that otherwise 

 single segment of the limb into two, whose greater lengths are parallel with, instead of 

 perpendicular to, the axis of motion at the radio-ulnar articulation ; and since the biceps is 

 attached to the radius, it really extends over three joints, and can act in three ways : at the 

 shoulder in extension of the humerus, at the elbow in flexion of the fore-arm, and at the 

 radio-ulnar articulation in supination or extension of the radius ; for, regarding these two 

 bones alone, supination is extension, and pronation is flexion. 



It will be seen, however, that the radio-ulnar articulation is not a joint, in the same sense 

 as is the shoulder or the elbow. These latter are constant or nearly so throughout the 

 vertebrate series, and are important anatomical characters ; they are strictly morphological 

 joints ; but the arrangement by which movement is permitted between the two bones of 

 the single segment fore-arm, exists in comparatively few species of mammalia, and these 

 such as in their approach to man, have their fundamental structure most modified in rela- 

 tion to the higher uses they are to perform. The radio-ulnar articulation is properly a 

 teleolugical joint, and as might be expected, its existence does not really interfere with the 



MEMOIKS BOST. SOC. NAT. HIST. Vol. I. 19 



