534 



H. F. Osboru 



tbat one cannot iindevstand how it was overlooked by Stieda. This 

 little bundle which represents the rudimentary corpus callosum , forms 

 a portion of the lamina terrainalis (figs. 22 and 23) and is about \'c 

 the diameter of the anterior commissure. It lies some distance be- 

 low the foramen of Monro and in vertical as well as in transverse 

 sections its fibres are seen to pass directly upwards in the thin 

 inner wall of the lateral ventricle so far as could be ascertained 

 none of its fibres pass back into the hippocampal region, as the 

 commissura cornu ammonis. Here it seems to differ from the 

 brain of Columba. The great reduction of the inner wall of the 

 hemispheres has already been suggested as the cause of the dimi- 

 nutive size of the corpus callosum in the birds. 



I can still find no evidence that the pars olfactoria is pre- 

 sent in the anterior commissure. It is wanting in the brain of the 

 young pigeon and in the most complete series of the brain of the 

 adult duck I see no trace of it. The olfactory lobes it is true are 

 extremely reduced in the birds (fig. 6, text) but these lobes are very 

 large in the chelonian brain, where I have also failed to find a dis- 

 tinct pars olfactoria , so it does not appear that this commissure is 



invariably developed in proportion 

 to the size of the olfactory lobes. 

 In horizontal sections however there 

 Iv is considerable evidence in Anas 

 as in Emys that the entire an- 

 terior commissure does not pass into 

 the postero-lateral or temporal region 

 of the brain. There is a well mar- 

 ked division which passes forwards 

 and upwards. This I doubtfully re- 

 presented as the pars olfactoria in 

 the turtle brain (Taf. XIII, figs. 16 



hem 



Transverse section of the brain of Anas 

 boschas, through the cerebral commissures. 



and 17), but I now think that it represents the pars frontalis, 

 or commissure of the lateral portions of the mantle of the hemi- 

 spheres. The main division of the anterior commissure represents 

 the pars temporalis. It passes beneath the nucleus in the centre 

 of the hemispheres which corresponds to the corpus striatum and 

 extends widely into the temporal lobe before its fibres scatter into 

 the cortex. 



