418 STUDIES IN ADVANCED PHYSIOLOGY. 



go down the animal scale the relative difference decreases 

 until in some of the lower vertebrates the fore-brain is the 

 smallest of the divisions. This is especially true in the 

 fishes, in which the mid-brain or optic lobes form the bulk 

 of this organ. The cerebellum and medulla, on the other 

 hand, do not decrease in the same proportion, but are rela- 

 tively large in all animals. The disproportionate size of the 

 human cerebrum makes possible those higher psychical 

 functions which belong to man alone. 



Convolutions. The cerebrum is divided by a deep 

 median fissure into two almost separate halves called the 

 cerebral hemispheres. Viewed from the top and sides the 

 surface of the cerebrum is thrown into deep convolutions or 

 gyri. While these convolutions occur in the brains of many 

 of the lower animals they are much deeper in the human 

 species, and there is even an increase in the depth as we 

 proceed from the lower to the higher races of mankind. 

 These convolutions find their explanation in the fact that 

 the surface of the brain is thereby materially increased. 

 Rough calculations on the actual surface of the average 

 brain show that it may reach an extent almost equal to 

 that of the larger portion of the trunk itself, but by means 

 of these foldings this surface is enclosed in the relatively 

 small cranium. The desirability for a large cortical surface 

 is apparent when it is remembered that the principal cells 

 concerned in sensation and volition are found here. While 

 these convolutions seem to run without any appearance at 

 regularity, and while in detail they do differ somewhat in 

 different individuals, their general plan is constant, and by 

 means of them spots on the cortex of the brain are local- 

 ized. A few of the principal convolutions only, need be 

 mentioned here. One of these is the large fissure of Syl- 

 vius, on each side of the brain which lies between the lateral 

 lobes of the brain and the main portion, running from the 

 base of the brain upwards and backwards towards the oc- 

 cipital region. Connecting with the fissure and running 



