Midbrain 71 



Not only are the anterior colllculi much larger in the rat 

 than in man, but the structural differentiation within them, 

 as just described, is much more marked. 



At the anterior boundary of the tectum, where it con- 

 tinues into the thalamus, lies the large posterior commissure 

 {commissura posterior), which appears as an anterior exten- 

 sion of the stratum profundum of the anterior corpora 

 quadrigemina (Pis. X\'I., XML, XXM.). This commissure 

 is apparently a rather fundamental structure as it is very 

 constant throughout the whole vertebrate series and develops 

 early in ontogeny. Nevertheless its relations and functional 

 significance are not yet clearly understood. At each side of 

 the commissure, its fibres spread out in a large mass of cells 

 which has been called the nucleus of the posterior commissure, 

 and which is continuous with an elongated cell mass in the 

 deep layers of the optic tract and with the deeper parts of 

 the anterior coUiculus. The nucleus of the posterior com- 

 missure receives optic tract fibres and gives rise to a large 

 part of the commissure itself in the rat. Papez is inclined to 

 believe that the commissural fibres rising in this centre 

 terminate exclusively in the region of the interstitial nucleus 

 or the nucleus of Darkschewitsch on the other side, thus 

 leading from the optic tract to the heterolateral interstitio- 

 spinal tract. Another important constituent of the com- 

 missure is probably made up of ascending vestibular fibres 

 from the lateral part of the medial longitudinal bundle, and 

 there may also be contributions from the posterior nucleus 

 of the thalamus (p. 82) and from the nucleus of Darksche- 

 witsch. 



