656 CEREBRAL COMMISSURES OF THE MAMMALIA, 



and developmental facts, as well as the histological structure as 

 described by Ramon y Cajal, seems to favour the view — then the 

 cleft between the mesial stria and the callosal gyrus would be the 

 anterior extremity of the Bogenfurche. So that for practical 

 purposes the opinion of Schwalbe (which is essentially identical 

 with those of Hertwig, Schmidt and Mihalkovics) that " the 

 Bogenfurche in its upper part becomes the upper boundary of the 

 corpus callosum " is for all practical purposes correct. Strictly 

 speaking, however, the callosal fissure must be regarded as a new 

 formation, because with the degenerate condition of the hippo- 

 campus is associated the practical absence of the " Ammonsfurche," 

 the callosal fissure being formed by the growth of the cortex above 

 the corpus callosum, causing it to bulge over the latter. So that 

 Professor Cunningham's statement* must be regarded as strictly 

 correct " that the Bogenfurche is transitory and is gradually 

 obliterated during the time the corpus callosum assumes shape." 

 The facts brought forward in this paper afford a perfectly 

 rational explanation of the circuitous course taken by the fornix 

 fibres in the higher mammalia, without resorting to any such 

 theory of the rotation of the brain as that advanced by Hillf — a 

 pure speculation utterly opposed to all the facts of development. 

 From an examination of the brain of a higher mammal, there is 

 no apparent reason wh}' the hippocampi should not be connected 

 like the rest of the tempoi-al lobe by the anterior commissure 

 fibres. But just as by a comparison with the condition met with 

 in the Prototheria, the reason for this is apparent, so also is the 

 meaning for the course of the anterior pillars of the fornix above 

 the foramen of Monro to reach the optic thalamus self-evident. 



* " The Surface Anatomy of the Primate Cerebrum " Cunningham 

 Memoirs, Roy. Irish Acad. No. vii. 1892. 



t Plan of the Central Nervous System, 1885. 



