Chap. XXIII.] OF THE HUMAN BRAIN. 453 



small proportion to the fibres passing from one part of 

 the surface to another." 



a. Commissures connecting similar parts in the two 

 Hemispheres. — These are generally spoken of under the 

 name of ' transverse ' Commissures. They include the 

 Corpus Callosum, the Anterior Commissure, together with 

 the Middle and Posterior Commissures. A part of them 

 have been hitherto referred to, in the quotations from 

 Broadbent's descriptions, as * callosal ' fibres. 



The Corpus Callosum is by far the largest and most 

 important of all the commissures. When the two Cere- 

 bral Hemispheres are separated it may be seen as a broad 

 band of fibres extending from the one to the other. Its 

 antero-posterior diameter is over three inches, whilst 

 laterally it extends into the substance of each Hemisphere, 

 where it forms the roof of the ' lateral ventricles.' On 

 section it is seen to be thickened at each extremity 

 (fig. 162, 27, 28). 



Various notions have been held by older anatomists as 

 to the distribution of the fibres of the Corpus Callosum 

 which need not now be discussed, though it may be 

 mentioned that Foville thought its fibres served to bring 

 the Crus of one Hemisphere into relation with that 

 of the other ; and that, according to Gratiolet, its fibres 

 suffice to bring the Crus of one side into connection with 

 the convolutions of the opposite Hemisphere. The inves- 

 tigations of both Meynert and Broadbent, however, lead 

 them to believe that the first of these \iews is altogether 

 erroneous and that the second, if at all, is onl} very 

 partially true, since in the main the fibres of the Corpus 

 Callosum serve to unite similar Convolutions in the two 

 Hemispheres.* Its fibres are not, however, distributed to 

 all alike, but only to some of them. And, as before 

 * " JoLirn. of Meiit. Science," Ap., 1870, p. 18. 



