Midbrain 67 



upper edge of the pons and penetrate into It. These large 

 bundles gradually decrease in size as they descend, giving off 

 fibres to various parts of the brain, and analysis shows that 

 they are composed of several distinct tracts. In man the 

 medial fifth of the bundle is composed of fibres running from 

 the frontal cortex to the pontine nuclei — the fronto-pontine 

 tract — but this portion may be presumed to be very small 

 in the rat if it is present at all, as the frontal cortex is developed 

 to a very slight extent. The lateral fifth in man is made up 

 of fibres from the temporal and occipital lobes to the same 

 nuclei — the temporo-pontine tract. These two together form 

 the cortico-pontine tract. Between these lies the large cortico- 

 spinal tract, with which are mingled many cortico-hulbar 

 fibres. Many of the latter, however, are grouped in the mid- 

 brain into two distinct bundles, w^hich pass to the nuclei of 

 the cranial nerves. 



As might be expected, the basis pedunculi, consisting of 

 fibres of neocortical origin, varies in size in different animals 

 according to the development of the neocortex. It is largest 

 in man, where the neocortex is most highly developed, and 

 is considerably smaller in the rat. 



The tectum is the dorsal portion or roof of the midbrain. 

 In the lower vertebrates, it is moulded into two rounded 

 optic lobes, where terminate most of the fibres of the optic 

 nerve, while the posterior part of its ventro-lateral portion 

 contains a more or less massive auditory correlation centre. 

 In the mammals, the latter centre has acquired a more dorsal 

 situation and appears as a second, usually smaller eminence 

 behind the reduced optic lobe. Hence the optic lobes have 

 come to be known in mamrnals as the anterior {superior) 

 corpora quadrigemina or anterior (superior) colliculi, while the 

 new eminences are the posterior (inferior) corpora qicadrigemina 

 or posterior (inferior) colliculi. In the rat and other lower 

 mammals, the latter are considerably smaller than the 

 former, while in man they are more nearly alike in size. This 



