716 THE MUSCULAR SYSTEM 



On the other hand, the prehensile or grasping movement of the tail of the 

 opossum, or the tails of western-hemisphere monkeys necessitates an extreme 

 adaptation on the part of individual muscle bundles and their attachment to 

 the caudal vertebrae. Similar specializations are found in the writhing tail of 

 the cat group. The wagging movement of the tail of the dog or the swishing 

 motion of the tails of cows, horses and other mammals is the result of the 

 activities of the Mm. abductor caudae internus and abductor caudae externus 

 which appear to be derivatives of the hind-limb musculature. 



b. Development of Muscles of the Head-pharyngeal Area 



1) Extrinsic Muscles of the Eye. The extrinsic muscles of the eyeball are 

 one of the most constant features of vertebrate morphology. Six muscles for 

 each eye are found in all gnathostomes, innervated by three cranial nerves 

 as follows: 



( 1 ) M. rectus superior — cranial nerve III, 



(2) M. rectus internus or anterius — cranial nerve III, 



(3) M. rectus inferior — cranial nerve III, 



(4) M. rectus externus (posterius or lateralis) — cranial nerve VI, 



(5) M. obliquus superior — cranial nerve IV, and 



(6) M. obliquus inferior — cranial nerve III. 



To these muscles may be added the Mm. retractor oculi of many mammals 

 and the Mm. quadratus and pyramidalis of birds. 



In the shark group, the muscles of the eye arise from three pre-otic somites 

 or head cavities, namely, the pre-mandibular, mandibular and hyoid somites 

 (figs. 253, 327A). The pre-mandibular somite, innervated by the oculomo- 

 torius or third cranial nerve, gives origin to all of the rectus muscles with the 

 exception of the Mm. rectus externus. The Mm. obliquus inferior also arises 

 from the pre-mandibular somite. From the mandibular somite, innervated 

 by the trochlearis or fourth cranial nerve, arises the Mm. obliquus superior, 

 while the hyoid somite gives origin to the Mm. rectus externus (Balfour, 1878; 

 Piatt, 1891; Neal, '18). A derivation of eye muscles from three pre-otic 

 somites or mesodermal condensations has been described in the gymno- 

 phionan amphibia by Marcus ('09), in the turtle by Johnson ('13), in the 

 chick by Adelmann ('26, '27), and in the marsupial mammal, Trichosurus, 

 by Fraser ('15). For extensive references regarding the eye-forming somites 

 or mesodermal condensations, see Adelmann ('26, and '27). 



Various disagreements, concerning the presence or absence of the various 

 head somites and the origin of the eye muscles therefrom, are to be found 

 in the literature. Regardless of this lack of uniformity of agreement, it is 

 highly probable that the premuscle masses of tissue which give origin to the 

 eye muscles in the gnathostomous vertebrates, in general, adhere closely to 



