CEREBRAL COMMISSURES TX THE VERTEBRATA. 483 



sense of the morphological importance attaching to them, because he regarded the dorsal commissure as 

 tlie " corpus callosnm," not having recognized the liippoeampus as such. Hence the fact that the fibres 

 have a slightly wider area of origin would mean little. 



The possibility of the existence of a corpus callosuni in tlie Rcptiiia in this sense has not hitherto been 

 suggested, so far as I am aware ; but it is patent that no other forui of true corj)t>s cfillosiim ran exist in 

 the ReptiUa thnn this, as yet, hypotlietical commissure of the lateral palliunt. 



I shall have occasion to return ineidcnrally to this point later on. At the same time 1 eainiot too 

 strongly insist upon the fact that, even granting the existence of these true homologucs of the corpus 

 callosuni iu the reptilian In-aiu, the vast majority of tiie fibres of tlie dorsal commissure are unquestion- 

 ably derived from the liippoeampus, and therefore cannot l)c regarded as part of the true corpus 

 callosuni. 



In the year 1887 Belloiiei |)nblishcd a nieinoir on the cerebral etniiniissures in the Amphibia and 

 Reptilia *, in which he stated that in his " tratto superiore della couunissirra anteriore " decussational fibres 

 were interiuingled with true commissural fibres. The commissure to which he thus refers is the 

 commissuru ilorsalis or liippoeampal commissure, which he regai'ded as part of the anterior couimissure — 

 an utterly untenable view. The existence of the decussational fibres which pass obliqueh' from the 

 descending hippoeampal fibres of one side to the opposite side of the brain has been since observed by 

 many investigators, and may be readily demonstrated by the method of Weigert, especially in the 

 Saurians. These fibres are represented in the accompanying drawing (fig. 20, a, p. 473), which has been 

 made, with the aid of the camera lucida, from a coronal section of the brain of Jlydrosaurus just in front 

 of the commissuru ventralis. I am utterly unable to offer any explanation of this extraordinary course for 

 fibres obviously belonging to the fornix system: nor am I able to find iu the Mammalia any fibres which 

 can be compared with these decussating bundles. No other writer, so far as I am aware, has offered any 

 feasible explanation of this arrangement. Ramon y Cajal represented the fibres in his figure to which 

 I have referred above; and it may be possible — although improbable — that they may represent the 

 continuation of the fibres which he indicates as coming from the neopallium beyond the hippocampus. 

 In other words, the;/ may be pallial projection fifjres. I do not, however, place any eonfidence in this 

 hypothesis, and can only mention this vague possibility in lieu of the utter lack of suggestion concerning 

 these decussating fibres. A knoAvlcdge of the alleged existence of such fibres should impose upon 

 investigators great caution in attempting to interpret the reptilian fornix. 



It is unnecessary to write at any length regarding the commissures in the cerebral hemispheres of the 

 Bird. A study of the process of development of the chick's brain and a comparison of the histological 

 structure of the mesial wall of the hemisphere in the adult show that iu birds a hippocampus of the 

 chelonian type makes its appearance but never attains to large dimensions. From it a small hippoeampal 

 commissure springs, as in the reptile. 



To speak of a " corpus eallosum " in the bird, such as Bryce aud Warrington have done even so 

 recently as 1898 (Report of International Physiological Congress, Cambridge, 1898), is a gratuitous 

 assumption ; because the dorsal commissure is unquestionably hippoeampal. The same arguments 

 concerning this homology which 1 have applied to the reptilian condition might be equally appropriately 

 employed in this case also. 



* "SuUe Comniissure cerebrali auteriori degli Anfibia e doi Eettili," 'Slcm. del. Real, -iccad. del. Sci. dell' Istituto 

 di Bologna, Ser. iv. Tom. viii. IS)"?". 



