Physiology of the Central Nervous System of the Bird. 177 



The other prominent features are the large optic lobes, on the surface 

 of which shining white fibres can be seen, and also the excessive size of 

 the optic tracts, the diameter of which is equal to that of the spinal 

 cord in the dorsal region. 



The general plan of the tracts of the central nervous system is as 

 follows : 



I. From the hemispheres tracts arise which undergo a descending 

 degeneration, and terminate in the thalamic and mesencephalic region.. 

 There is no direct connection of the cerebral hemispheres with the 

 spinal cord. 



II. A well marked tract arises in the mesencephalon, undergoes an 

 ascending degeneration, and can be found to terminate in the substance 

 of the hemisphere. 



III. The mesencephalon is the site of a complex system of fibres, 

 which can be grouped as follows : 



(a) The ascending tract to the hemispheres alluded to above (Tr. 

 mesencephalicus striatus). 



(/?) Arciform fibres decussating in the middle line and reaching 

 downwards into the spinal cord (Forel's and Meynert's foun- 

 tain decussation). 



(7) The optic tracts and various commissural fibres connected with 



this. 



(8) A tract originally described by Perlia, and called by him the 



median optic bundle. 



This tract can be well seen after all lesions involving the optic lobes 

 as a well defined degenerate bundle, situated on the inner side of the 

 dorsal aspect of the optic tract, and ending in the ganglion isthmi, 

 which is situated in the optic lobe at the junction of that body 

 with the cerebellum and pons. Peripherally, Wallenberg found that it 

 could be traced to the ganglion layer of the retina of the opposite 

 eye, and Perlia, in his original description, observed the tract as a 

 degenerated bundle found after enucleation of an eye. 



IV. The commissural system, including 



(a) The anterior commissure. 



([$) The posterior commissure. 



(y) The commissure of the roof of the aqueduct (lamina commis- 



suralis mesencephali) which contains the large cells thought 



to be the mesencephalic nucleus of the trigeminus. 

 (S) The small pallial commissure. Hippocampal commissure of 



Elliot Smith ; commissura anterior et posterior pallii of 



Edinger. 



V. The tracts of the spinal cord, which have broadly the usual 

 ascending and descending course. 



