RECURRENT BRANCHES OF ABDUCENS NERVE 389 



is shown by the separation of the fibers from the brain (probably 

 after loss of the nerve cells in the abducens nucleus) and their 

 digestion by phagocytes. The distribution, of those fibers long 

 enough to make their destination sure, is to the anterior spinal 

 muscles, the branchial musculature, or the dorsal muscles of the 

 head. They may arise from the same roots as the abducens 

 proper, or by separate but closely adjacent rootlets. They are 

 absent in embryos whose eye muscles predominate over the 

 occipital muscles, either by proximity or precocity, but the 

 position of the first abducens roots on the floor of the medulla 

 is also a factor, as the more caudally these arise, the more often 

 are recurrent branches to be expected. Among the mammals 

 studied, man alone shows the abducens roots emerging in order 

 from behind forward, with the oculomotor appearing later than 

 the abducens. In sheep, pig, and rabbit the growth wave is 

 reversed, beginning with the oculomotor and extending caudally. 

 In man, then, the earliest abducens roots are caudal, nearer 

 to the occipital muscles, in the other mammals they are cranial, 

 approaching the eye-muscle mass. In birds and reptiles the 

 eye and its muscles are not only precocious, but the muscles 

 are actually displaced caudally by the large eyeball, so that 

 their influence on the emerging abducens fibers extends far 

 caudally and only the fibers close to the hypoglossal roots turn 

 caudally. These are soon lost, and the permanent abducens 

 shows no recurrent branches. In fishes the abducens roots 

 arise from more caudal neuromeres, and occasionally some fibers 

 turn caudally, but only as a rare anomaly. 



