446 



HISTORY OF THE HUMAN BODY 



These three pairs are small and very special nerves, having 

 no other distribution than the six muscles of the eyeball. 

 They are thus exclusively motor, and on the theory that the 

 cranial nerves, excepting, perhaps, those of the preceding 

 group, represent modified spinal nerves, seem to correspond 

 to the ventral roots of three original nerves, the sensory roots 

 of which are either lost, or, more probably, contained in the 



Mes 



FIG. 124. The Nervus terminalis of the selachians. [After LOCY.] 



(a) Dorsal view of the brain of the dog-fish, Squalus acanthias. (b) Horizontal 

 section through the anterior part of the same, showing the origin of Nervus ter- 

 minalis. 



Nas, nasal capsule; Olf, olfactory lobe; N. ter, Nervus terminalis; gl, its ganglion; 

 "Tel,, telencephalon; Di, diencephalon; Mes, mesencephalon; Met, metencephalon; 

 Myel, myelencephalon. 



sensory elements of adjacent cranial nerves such as Trige- 

 minus or Facialis. The fact which suggests this hypothesis 

 most strongly is the strictly metameric character of their 

 field of distribution, namely, the eye muscles themselves, as 

 is shown by their developmental history. In selachian em- 

 bryos, which have preserved this early history more completely 

 than have the higher forms, there develops in the head a series 

 of myotomes, similar to and continuous with those of the 

 trunk. Some of them soon atrophy, but the first three fold 



