246 H. F. Osborn 



There is a question whether the bundle diverging from the corpus 

 callosum in the Sauropsida represents the commissure of the cornu 

 ammonis or the posterior pillar of the fornix. The homology of the 

 commissurae inter lobular is in the fish brain is doubtful. 

 Does it represent one or both of the Amphibian bundles? The Dip- 

 noan brain will probably give us the solution of this question. 



With these limitations, the following are the results reached 

 at present: (1) In the brain of the fishes (Teleosts), the olfactory- 

 lobes are united with each other by an olfactory commissure, which 

 probably is homologous with that part of the pars olfactoriain 

 the mammalia which supplies the olfactory lobes. Within this, are 

 the inner olfactory tracts which arise from nuclei of uncertain 

 position and. crossing each other in front of the olfactory commissure, 

 enter the olfactory nerves forming an internal and posterior root. 

 The cerebral lobes are united by one or more bundles of fibres which 

 constitute the commissura interlobularis. (2) In the Amphi- 

 bians the cerebral commissures lie behind the foramina of Monro. 

 They consist of an upper and lower bundle. The former is much 

 the larger. It passes upwards and forwards behind and above 

 the foramen and gives the commissural supply of the dorso- medial 

 portion of the hemispheres. It is the corpus callosum. The 

 lower bundle is the anterior commissure. Its main division 

 is the pars olfactoria which supplies a part of the brain stem, 

 the ventro - lateral portion of the hemispheres, and probably the 

 olfactory lobes. Its lesser division, supplying the lower posterior por- 

 tion of the hemispheres is the pars temporalis. The inner ol- 

 factory tracts arise near the anterior commissure, cross each 

 other, and pass into the olfactory lobes. (3) In the Reptiles 

 (Chelonia), the cerebral commissures lie below and slightly in front 

 of the foramina of Monro. The larger upper bundle ascends in front 

 of the foramen and divides. The anterior division, or corpus cal- 

 losum supplies the whole of the inner mantle. The small posterior 

 division supplies the inner fold of the hemispheres and represents a 

 portion of the fornix. The lower bundle is the anterior com- 

 missure. It consists of an upper tract the pars olfactoria, the 

 distribution of which is doubtful: and of a lower tract the pars 

 temporalis which is distributed as in the mammalia. In connec- 

 tion with the corpus callosum is a bundle representing a portion of the 

 fornix. (4) The position of the cerebral commissures in the Birds 

 (Columba) is similar to that in the Reptiles : The upper bundle is very 



