MORPHOLOGY OF THE LIMB-MUSCLES. 261 



and on the deeper sui-face of tlie latissimus dorsi are its occasional scapular por- 

 tion and the teres major, all of which are formed in the deeper parts of this 

 layer. The muscles of this layer are frequently found to some extent united 

 together either as varieties in man, or as regular structures among some of the 

 lower animals, and it is interesting to note that most of the varieties hitherto 

 described, seem to be reversions so far towards a uniform tj-pe. Thus in Lepi- 

 dosiren, the pectoralis major and latissimus dorsi are blended together at their 

 margins, so as to form one continuous muscle ; and in man the frequently reciu-- 

 ring axillary arches or loops of muscular iibres which pass from one muscle to 

 the other across the axillary space may point to an imperfectly segmented condi- 

 tion. Again, the pectoralis major is always united in man with the superficial 

 fibres of the external oblique, but in the crj'ptobrancli it is continuous with it m 

 its entii'e thickness. The cleido-mastoid, which in man is in close relation with 

 the clavicular portion of the pectoralis major, being- only separated from it by 

 the clavicle rmis with the anterior part of the trapezius into the deltoid in those 

 animals in which the clavicle is deficient, fonning a mastoido-hmueral muscle. 



Tlie trunk-limb muscles of the deep layer, fonn a much less complete layer 

 than those of the superficial. They are attached to the shoulder-girdle in two 

 sets. In front of tlie glenoid cavity, and attached to the coracoid, is the costo- 

 coracoid muscle (in man called subclavius fi'om its more common place of 

 attachment) ; and behind the glenoid cavity, attached to the scajiula, are the 

 costo-scapular muscles, comprising the serratus magnus, and the levator scapulse. 

 Besides these there is the slender omo-hyoid from the front of the hyoidean arch 

 to the upper border of the scapula. 



In the lower limb the ventro-aiipendicular muscles are represented for the 

 most part by the oblique abdominal muscles, only those muscles which pass over 

 the girdle to the limb being fomid distinct. The gluteus maximus most closely 

 represents the latissimus dorsi, and is similarly inserted into the postaxial border 

 of the limb. The pectoralis corresponds most nearly ■with the gracilis, and the 

 trapezio-deltoid with the anterior part of tlie external oblique and pectineus. 

 Tlie gracilis and jiectineus arise near each other, and are inserted into the pre- 

 axial border of the limb. The pectineus, however, seems to consist of two i^arts, 

 of which the inner may be related to subclavius, and the outer to great jiectoral 

 find deltoid. 



All the other muscles of the limbs are intrinsic, and in their relation to the 

 several segments are arranged in groups which may in the main be compared 

 homologically with each other. These groups are situated chiefly on the ventral 

 and dorsal asjiects, or on the preaxial and postaxial borders of the limbs, and the 

 muscles of which they are composed are respectively flexors and extensors, or 

 jirotractors and retractors. 



In attempting, however, to institute a homological comparison between the 

 individual muscles of the various segments in the two limbs, it will be necessary 

 to bear in mind, first, the great difference in the attitude of the liones of the 

 proximal segments, the flexor surface in the upper limb being tlll•o^\^l forwards 

 as if by an outward twist, and in the lower limb the flexor surface being turned 

 backwards as if by an inward twist of the proximal segments : and further, the 

 preponderance in the second segment of the lower limb of the preaxial bone 

 (tibia) over the postaxial (fibula) as opposed to the greater development of the 

 upper end of the postaxial bone of the arm (ulna) over the preaxial (radius). 

 These differences are apt to lead to confusion and difficulty, inasmuch as the 

 insertions of homologous muscles are in some cases veiy different in the two 

 limbs. In such cases the detennination of the homologies may be greatly 

 assisted by the observation of the more constant origins. For example, the 

 great extensor of the forearm is inserted into the postaxial ulna, while in the 

 leg the quadi-iceps is inserted into the preaxial tibia ; and yet few will doubt 

 the justness of the homological comparison between them. 



In the distal segments of both limbs the muscles, though more numerous, 

 1)ecome shorter and more imiform in their disposition, and hence the homology 

 becomes more apparent between the mu-scles of the hand and foot, than in those 

 situated nearer the root of the limbs. 



