MORPHOLOGY OF EYE MUSCLE NERVES HI 



the lateral eyes, however, which at that time were in close prox- 

 imity to one another did not degenerate but became attached 

 to, and at the same time migrated with newly differentiated 

 fibers, to the eye of the opposite side where they increased in 

 size while the original muscles of that side atrophied. In this 

 way they formed the present superior oblique muscle, the nerve 

 of which with its dorsal chiasma — primarily peripheral — still dis- 

 closes the early history of the transmigration of its muscle. Then 

 with the higher development of the retina of the lateral eyes 

 there came an enlargement of the centers within the brain, espe- 

 cially a greater development of the roof of the midbrain, which 

 extended backward over the place of emergence of the trochlear 

 fibers and covered it, so that a part of its root as well as its 

 chiasma was shoved backward and at the same time enclosed 

 within the wall of the brain. The lateral eyes were not restricted 

 to that dorsal muscle derived from the second myomere of the 

 opposite side but very soon acquired connections with more 

 ventral musculature innervated by the oculomotor and abdu- 

 cens, and, as the eyes moved into a ventral position, the muscles 

 became enlarged and differentiated. 



In support of this hypothesis, Fiirbringer advances the follow- 

 ing considerations: First, in defense of the assumption that the 

 parietal eye once possessed a musculature of which today there 

 is no evidence (with possibly the exception of the unconfirmed 

 case in mammals mentioned by Nicholas ('00) who claimed to 

 demonstrate rudimentary striated muscles in the pineal region 

 of the ox), he argues that the absence of parietal muscles today 

 by no means proves that they never existed. Many skeletal 

 structures have existed in fossil species without leaving a trace 

 in modern vertebrates. Typhlichthys has no eye muscles but 

 its ancestors must have had them. The assumption of an anti- 

 meric transmigration of the hypothetical ancestral eye muscles 

 has met both opposition (Dohrn '01) and support (Gaskeil '01). 

 In reply to the opposition, Fiirbringer cites many cases of the 

 extensive migration of muscles in all directions within the verte- 

 brate body. Many instances of the transmigration of muscu- 

 lature on the ventral side of the body are known, as for example 

 the muscles innervated by the facialis, vagus and hypoglossus, 



