152 



MR HAROLD AXEL HAIG ON THE 



the grey matter surrounding the aqueduct shows mesioventrally the nuclei of the 3rd 

 nerve, and ventral to the latter the posterior longitudinal bundles are fairly obvious. 

 The crustse are of less extent than those of the former section. 



The posterior commissure of the brain shows dorsal to the aqueduct of Sylvius, and 

 is of moderate antero-posterior dimensions. 



Points of comparative value in connection with the medulla, pons, and mid-brain 

 are as follows : 



(a) The disproportion between the transverse and antero-posterior diameters of the 

 medulla, the former being about twice or two and a-half times the length of 

 the latter. 



(6) The relatively large proportion of grey matter in all these regions, much of it, 

 however, being composed of purely neuroglial tissue. 



(c) The aberrant shape of the olivary nucleus and the outward displacement of the 



whole olive. 



(d) The marked development of the restiform body and its early distinction in a 



comparatively low region. 



(e) The late formation of the crustse, the mid-brain being reached before these are 



well defined. Other minor points will be made out by reference to Plate I., 

 and comparison of the figures with sections of normal human material. 



V. THE HISTOLOGY OF THE OPTIC THALAMUS. (Text-fig. 4.) 



The main features to be made out from a vertical (sagittal) section of the optic 

 thalamus consist mainly in the relative distribution of grey and white matter ; the 

 white matter occurs in two main masses an external, thick superiorly and thinning off 

 towards the anterior aspect, and an internal oblique mass which divides the internal 



FIG. 4. A mesial sagittal section of the optic thalamus (semidiagrammatic). x 4 times. 



w.f. Superficial layer of white fibres into which septa of neuroglia pass from the deeper grey matter. 

 n.n.n. Groups of large nerve-cells in the grey matter. 



g.m. Central grey matter. The internal mass is not sharply differentiated into grey and white 

 matter : small nerve-cells appear scattered throughout the;former. 



(ROY. soc. EDIN. TRANS., VOL. XLVIII., 860.) 



