$6 Journal of Comparative Neurology. 



are normal. The decussation occurs in the posterior medullary 

 velum. From this point onwards the course of the nerve is 

 anomalous. It is the rule for the nerve to make its exit at the 

 level of the decussation, either from the dorsum, or occasion- 

 ally from the side, of the velum. In this case, however, the 

 nerve fails to do so ; instead, it turns upon its course, and runs 

 forwards and outwards, traversing first the velum, then the base 

 of the posterior quadrigeminal bodies. Its exit occurs in the 

 superficial sulcus of the midbrain, situated between the poster- 

 ior quadrigemina above, and the lateral convexity made by the 

 inferior fillet below. 



The explanation of this variation is coupled, in the present 

 state of our knowledge, with considerable difficulty. If we 

 adopt the grouping of variations suggested by Cunningham, 

 we shall, provisionally at any rate, place this among the " pra- 

 gonic " variations, the unclassified residuum. In the first place, 

 it corresponds with no stage in the normal embryogenesis of 

 the fourth nerve in the human species. Nor does it, so far as 

 we know, represent an atavistic condition, for it is a striking 

 fact that the essential relations of the internal course of the 

 nerve are identical in all vertebrates. 



The anomaly appears, therefore, to be inexplicable, or for- 

 tuitous. Nevertheless, there are certain circumstances which 

 indicate that it may afford a possible clue to the original course 

 of the Trochlear nerve. The foetal segmentation of the brain 

 is violated by this nerve, in that its origin and exit are separated 

 by one of the primary constrictions of the foetal brain. It has 

 its nucleus in the adult midbrain, or second cerebral vesicle, 

 while it leaves the brain in the anterior portion of the foetal 

 third cerebral vesicle, the so-called Isthmus Rhombencephali. 

 This origin and exit are constant throughout the vertebrate 

 series. If the posterior limit of the adult midbrain coincides 

 with that of the so-called mesencephalic segment of craniolo- 

 gists, it is evident that certain of the internal relations of the 

 fourth nerve must have been secondarily acquired. The seg- 

 mental nerve must, of course, leave the central system from 

 that segment in which it originates. This throws the difficulty 



