THE MUSCULAR SYSTEM 223 



In mammals, corresponding to the great difference of struc- 

 iire shown among the different Orders, there is a great di- 

 versity in the appendicular musculature, but if there be taken 

 'or comparison with the above any of the more primitive 

 pentadactylous quadrupeds, such as a marsupial, a rodent or a 

 lower primate, the majority of the muscles in the two can be 

 readily homologized (Figs. 59, 60). Laiissimus dorsi and 

 trapezius have greatly increased in extent ; the former, having 

 reached the mid-dorsal line, no longer possesses a free dorsal 

 margin anteriorly, and posteriorly shows no trace of the primi- 

 tive myotomic slips of which it was originally composed. A 

 slip, segmented off from its anterior edge, has become a sep- 

 arate muscle, with the name of teres major. The trapezius 

 extends from the occipital region of the skull, a point which it 

 attains in the higher salamanders, along the mid-dorsal line, to 

 a point considerably posterior to the scapula, where it overlaps 

 the latissimus. It may either be divided into three distinct 

 slips, anterior, middle and posterior, or may be in the form of 

 an unbroken sheet; and in climbing arboreal forms, like the 

 monkeys and apes, and in man, is of enormous extent, the two 

 covering the entire upper half of the back and prolonged 

 posteriorly into a median point like a monk's hood, whence the 

 alternative name of cucullaris, employed by European anato- 

 mists. From its anterior margin a bundle of fibers is set off 

 and becomes the sterno-cleido mastoideus, a muscle running 

 obliquely across the side of the neck from the anterior end of 

 the sternum to the skull just behind the ear, and conspicuous 

 in man. 



The deeper layer of extrinsic muscles, levator scapula? and 

 serratus anterior \_magnus~], have increased meanwhile, prob- 

 ably by the addition of intermediate slips that arise from the 

 myotomes between the two, and become in most mammals a 

 continuous layer, the primary metamerism being expressed in 

 its slips of origin, which form " digitations," or separate 

 pointed slips that arise from the successive ribs, or from their 

 equivalent processes in the cervical region. In man these 

 muscles are again separated into two by the failure of certain 



