320 



forming their floor by running on backwards, just as before bend- 

 ing down it had formed their ceiling. This pretended anterior 

 fold is consequently termed by Gall the mass of the union of 

 the inferior convolutions of the anterior lobes. The fornix is the 

 commissure of the posterior convolutions of the middle and of all 

 of the posterior lobe, and is called by Gall the mass of the general 

 communication of the brain. The lyre is the assemblage of the 

 filaments of union in the fornix. The pretended posterior fold 

 of the mesolobe is the commissure of the posterior internal con- 

 volutions of the middle lobe. The anterior convolutions of the 

 middle lobe, and some situated at the bottom of the great Jissure 

 of Sylvius, called by Gall the great Jissure between the anterior and 

 middle lobes of the cerebrum, give rise, by their union, to what is 

 called the anterior commissure of the lateral ventricles, but by 

 Gall the union of the anterior convolutions of the middle lobe. The 

 posterior commissure of the lateral ventricles cannot be traced to 

 the convolutions, but only just into the thalami optici, and is 

 therefore named by Gall the posterior commissure of the great 

 inferior cerebral ganglion. The middle commissure, for the same 

 reason, and on account of its softness, is called by him the soft 

 union of the great inferior cerebral ganglion. (Cut, p. 313. 316.) 

 Each of these points of union is proportionate to the parts which 

 it unites. Gall considers the origin of the converging fibres to 

 be in the superficial pulpy substance. 



The converging fibres of all these commissures, after lining the 

 interior of the two lateral ventricles, or great cavities of the cere- 

 brum, as Gall styles them, while he terms the third ventricle the 

 space between the great inferior cerebral ganglia, interlace with the 

 diverging fibres, and thus form a true tissue. (Cut, p. 313. No. 

 35, 36. 38, 39. ; p. 316. No. 33, 34. 36, 37.) 



The diverging fibres are then prolonged in the form of a 

 fibrous expansion. 



If the ventricles are opened, and their walls gently expanded 

 with the hand, or if fluids collect in them, as in hydrocephalus, 

 the tissue of diverging and converging fibres is at length lacer- 

 ated. After this, the expanding force acts upon merely diverging 

 fibres, and all the convolutions disappear; the brain becoming 

 expanded into a smooth bag. A convolution is thus proved to be 

 two fibrous layers, placed side by side, and very slightly united : 

 therefore, if air or water is impelled against the centre of a 



