BRAIN. 149 
groups, forces the hippocampus to the medial side of the hemisphere. 
Other modifications are better understood after a consideration of 
the commissures of the brain. 
The amount of gray matter in the pallium is evidently correlated with the mental 
powers of the animal, being greatest in the mammals. Here the nerve cells form 
a layer (cortex) on the surface of the neopallium. Increase in the number of 
these cells can be accommodated to some extent by increase in the size of the 
cerebrum, but the extent of this increase is limited, and in the higher mammals the 
amount of surface is increased by folding, so that the cerebrum is marked extern- 
ally by numerous fissures or sulci separating convolutions or gyri, as will be 
mentioned in the paragraphs on the mammalian brain. 
_ In order that the two sides of the body may work in harmony it is 
necessary that the right and left side of the central nervous system be 
te coe al 
Fic. 152.—Medial plane of brain of Ornithorhynchus, after G. Elliot Smith. ac, anterior 
commissure; bo, bulbus olfactorius; cm, commissura mollis; cl, cerebellum; e, epiphysis; 
fd, fasciculus dentatus; fi, interventricular foramen; h, hypophysis; kc, habenular commis- 
sure; /p, lobus pyriformis; mc, corpus mamillare; md, medulla oblongata; 2, nodulus; ol, 
olfactory lobes; of, olfactory tubercle; pal, pallium; pc, posterior commissure; fc, tuber 
cinereum; v, velum medullare; vmo, motor root of fifth nerve; vm, maxillary of fifth. 
connected. This is accomplished in the spinal cord by nerve fibres 
which pass above and below the central canal from one side to the other. 
In the brain these commissures are more localized. Then there are 
longitudinal fibre tracts in the brain, some of which are continuous with 
the columns of the cord already mentioned. Only a few of these connex- 
ions, which are more numerous in the higher than in the lower verte- 
brates, can be mentioned here. 
Most constant and important of the commissures are the following: 
