142 JOUHNAL OF COMI'AUATIXE Nei?K()1.()GY. 



cells of the oculo-motor nidulus, the posterior connects with 

 the cell-processes of the pale ganglion. None of the fibres 

 of the commissure can be traced directly into the main 

 sensory tract; in Ratia they can be followed into the peri- 

 pheral white substance of the tectum opticum. According 

 to the investigations of Goronowitsch ['% p. 1^51] on the 

 brain of Acipeiisc7-, the posterior commissure consists of 

 three parts. The distal portion is formed of medullated 

 fibres which, in sagittal sections, can be traced to the base of 

 the mesencephalon. The proximal portion is made up of 

 fine medullated fibres which appear to come from a group of 

 small nerve-cells, dorsal to the bundle of Meynert; the 

 further course of this portion he was unable to trace. The 

 third portion is divisible into three parts: the first consists of 

 a granular fibrous structure, in whose periphery well-defined 

 fibres of the optic tract may be se6n; the second of two or 

 three rows of cells, the processes of which are sent out in the 

 direction of the peripheral layer; the third is made by the 

 inner surface of the tectum. In the proximal portion of the 

 tectum the granular layer becomes thinner. Median to this, 

 large, rather thickly scattered ganglion-cells are to be found. 

 The small processes of these cells run upwards to the surface 

 of the tectum, and appear to enter into connection with the 

 optic fibres. Auerbach ['% p. 373 et seq.] has made a series 

 of investigations on the structure of the posterior commissure 

 in the teleosts and in the higher vertebrates; but the con- 

 clusions embodied in his paper need not be mentioned in 

 addition to the above. 



Pawlosky says: "The so-called posterior commissure 

 (in mammals) consists of crossed nerve-fibres descending 

 from the brain to the tegmentum of the crus." They origi- 

 nate: I. In the habenular ganglia. 2. In the frontal lobes of 

 the brain, through the anterior peduncle of the thalami. 3. 

 In the temporal lobes, through the lower peduncle. 4. Per 

 haps in the thalami themselves. 



This commissure, throughout all the vertebrate types, is 



