No. I.] OPTIC LOBE LAYERS IN LOWER VERTEBRATES. 9 



layer Z>, and a great portion is reflected so as to enter the cell- 

 layer F of the opposite lobe. The reflected fibres from the 

 opposite sides together constitute the commissnra tecti optica. 

 So far, the nerve-impulse through the fibres has been sensory. 

 In layer /^the conversion into, or relation with, motor impulses 

 must take place, because we find fibres and cells from this layer 

 supplying directly the oculo-motor and trigeminal nerves, and 

 indirectly, probably, through the nucleus vnii^uus, the trochlear 

 and abducens. The presence, constant in all the forms, of the 

 large ganglion cells of the mesencephalic trigeminal nucleus 

 confirms this hypothesis. In the layer /% therefore, we arc to 

 look for reflexes of the III, IV, V, and VI nerves (Fig. 12, 

 diagram). And consequently the eye-muscles and other parts 

 supplied by those nerves may be excited into activity in a reflex 

 manner. Furthermore through the relations these tracts have 

 with the centres in the Medulla, this may serve to explain the 

 co-ordinated reflex movements exhibited by animals in which 

 the cerebrum has been removed. I have also observed fibres 

 which appear to show that the tracts coming from the cerebrum 

 enter into the layer F. In this case the cerebral voluntary 

 impulses would also run through this tract. This point, how- 

 ever, must be confirmed by further observation. 



This is of course a tentative hypothesis of the complex re- 

 lations of the optic lobe layers to each other, to the cerebrum 

 and to the origin of the cranial nerves ; it is, nevertheless, along 

 a line of observation which has not to my knowledge been 

 attempted before, and which it is evident will lead to definite 

 results. 



Princeton, August, 1889. 



BIBLIOGRAPHY. 



Edingep. Untersuchungen iiber die Vergleichende Anatomic des Ge- 



hirns. I. Das Vorderhirn. Frankfort, 1888. 

 OSBORN. A Contribution to tlve Internal Structure of the Amphibian. 



Brain. Journal of Morphology. Vol. II, No. i, July, 1888. 



