244 H. F. Osborn 



forwards in the outer wall halfway towards olfactory lobes. This 

 is certainly not a region of distribution resembling that of the fornix. 

 But a more positive objection to Rabl-Rückhard's suggestion is the 

 fact that a corresponding bundle is observed in some of the Mam- 

 malia, I may instance Ca via, in which the fornix is a well devel- 

 oped and entirely distinct structure. I look for the discovery of the fornix 

 in the Reptilia in connection with the bundle which I have compared 

 with the commissura cornu ammonis, which in fact is usually 

 considered a part of the fornix. Eabl-Rijckhard's observations upon 

 the lower bundle are that this is a single commissure of the brain 

 stem which he calls the pars temporalis. I find that the postero- 

 inferior fibres of the lower bundle alone pass into the temporal region. 



Birds. 



My studies of the cerebral commissures in the avian brain although 

 less complete are consistent with the facts observed in the Chelonia, 

 and confirm the interesting observation of Meckel that a rudiment 

 of the corpus callosum is present. 



The brains of pigeon nestlings were selected for study and un- 

 fortunately proved to be too embryonic to show the commissures satis- 

 factorily. In general the type is that of the reptilian brain somewhat 

 exaggerated. The upper bundle is much smaller than the lower. 

 It divides as it ascends in the reduced inner mantle, the anterior 

 division is very small and soon lost sight of, it represents the cor- 

 pus callosum. The posterior division is proportionally larger and 

 passes backwards roofing the foramen of Monro as in the Turtle, 

 and representing the commissura cornu ammonis. The lower 

 bundle is strongly developed and has been described by all previous 

 authors as the anterior commissure. It extends widely outwards 

 through the corpora striata, as described by Meckel, and is distri- 

 buted in the temporal region. I have observed no bundle extending 

 forwards, and at present this commissure seems to me to represent 

 merely the pars temporalis. In a more adult brain the pars ol- 

 lactoria may be found, but without doubt, in a rudimentary develop- 

 ment. How far the pars temporalis supplies the brain stem proper 

 is an unsettled question. It will be observed that the proportions 

 of the upper and lower bundles correspond with the thinning of the 

 inner mantle, and the extension of the temporal region. The olfac- 

 tory lobes are very small, and this may explain in part the absence 



