THE MUSCULAR SYSTEM 



267 



into dotsal and ventral moieties. Further separation and displacement 

 followed the enlargement of the optic and otic vesicles. In this way, 

 eventually, two sets of muscles, one dorsal and one ventral, were brought 

 into close proximity to the enlarging optic vesicles with which they finally 

 became functionally associated. 



During the phylogenetic transformation of myotomes into eye muscles 

 noteworthy changes come about in their nerve relations. While the first 

 myotome retains throughout phylogenesis its primary connexion with 

 the oculomotor nerve, the nerve of the second myotome, the trochlearis, 

 acquires a dorsal chiasma and retains connexion only with the dorsal 

 moiety of the myotome which becomes the superior oblique muscle. 

 There is no satisfactory explanation of the appearance of this chiasma. 

 The cells which by their outgrowth produce the fibers of the trochlear 



HINOBRAIN' 



'3RD SOMITE 



Fig. 221. — The mesodermal somites 1-3 from which the eye muscles develop, as 

 seen in a wax reconstruction of a 5-6 mm. Squalus (elasmobranch) embryo made by 

 Alton F. Chase. The somites are viewed from the left side. The anterior somite of 

 Miss Piatt later breaks up into mesenchyme. 



nerve are located in the base of the brain posterior to those which form 

 the oculomotor nerve. But the fibers of the trochlear nerve instead of 

 growing directly to the muscle of the same side, as do those of the oculo- 

 motor nerve, grow around the wall of the brain and cross above the brain 

 to connect with the superior obHque muscle of the opposite side. In the 

 days when it was assumed that there is a primary connexion between 

 nerve and muscle, some morphologists thought it necessary to assume that 

 not only the nerves but the muscles had interchanged sides — a sort of 

 internal leap-frog for which there is not a particle of evidence. But even 

 our present knowledge that muscle and nerve become connected with 

 one another secondarily gives us no clue as to how the chiasma was formed 

 during phylogenesis. 



Another difficult problem is presented in the nerve relations of the 

 external rectus muscle, which is formed from the ventral moieties of the 

 second and third mvotomes. The nerve of the external rectus muscle is 



