178 Anatomy and Physiology of Nervous System of the Bird. 



Without entering into detail, some points in connection with the 

 tracts of the cerebral hemisphere should be mentioned. 



(i) The Tr. septomesencephalicus forms a prominent feature on the 

 mesial wall of the hemisphere, where it is seen as a fan-shaped expan- 

 sion of white fibres. It rapidly converges to a well defined bundle, 

 which turns laterally, and is seen on the ventral aspect of the brain as 

 a band of fibres situated between the optic lobes and hemispheres. 

 Sections show that the anterior lobe of the brain contributes to its 

 formation, and that it terminates in the epithalamic region. Whilst the 

 main mass of its fibres pass in front of the anterior commissure, a dis- 

 tinct band passes posteriorly to that commissure to end in the region 

 of the ganglion habendulae. This part represents the fornix. It also 

 gives origin to fibres of the " pallial " commissure, and is in connection 

 with the optic tract by a well defined bundle. 



(ii) The middle region of the hemisphere especially, but also the 

 remaining parts to a less extent, give origin to the tracts which ter- 

 minate in the thalamus and mesencephalon respectively, viz., the Tr. 

 striothalamicus and Tr. striomesencephalicus. 



(iii) The expanded posterior part of the hemisphere is the site and 

 origin of three large tracts, viz., the anterior commissure, the Tr. 

 occipitomesencephalicus, and a great associational bundle binding the 

 anterior and posterior parts of the brain together, and called the fronto- 

 occipital tract. 



In considering the physiological significance of these tracts, their 

 anatomical distribution indicates the paramount importance of the 

 sense of sight in the bird. 



The optic tracts and lobes are enormously developed, and the latter 

 have connections with all parts of the central nervous system, being 

 thus a reflex centre of the highest importance. The well-developed 

 posterior parts of the hemisphere, with their connections with the 

 mesencephalon, illustrate in this animal the first formation of a higher 

 cerebral visual centre. 



Further, not only is there marked deficiency of sight in the opposite 

 eye after injury to one optic vesicle, but the same symptom is noticed 

 after injury to any part of the cerebral hemisphere. The defect of 

 sight is most marked after removal of the whole hemisphere, or of the 

 occipital portion ; but we are of opinion that distinct amblyopia fol- 

 lows a superficial lesion chiefly involving the pallial tract, or after 

 removal of the fore part of the brain, which, as mentioned above, is 

 connected with both this tract and with the occipital lobe. 



Two excitable areas are found on the surface of the hemisphere : 

 One situated on fibres of the pallial tract, near the median plane, 



and stimulation of which gives, as was long ago described by Ferrier, 



constant contraction of the pupil of the opposite eye. 



