276 STUDIES IN PHYSIOLOGY 



hindbrains. The relatively small optic lobes can be seen by 

 lifting up the cerebral hemispheres from the cerebellum. 



Comparison of Human and Frog Brains. The striking 

 contrasts Decween these two brains are these: First, in 

 the human brain the cerebral hemispheres and cerebellum 

 are enormously developed, and both are characterized by 

 the presence of convolutions. In the second place, while the 

 olfactory and optic lobes are proportionately large in the 

 amphibian brain, in man they are relatively small. And 

 finally, the two brains differ widely in the relation to each 

 other of the three regions '(fore-, mid-, and hindbrains). In 

 the frog's brain they lie in a straight line, one in front of 

 the other; the axis of the human brain, on the contrary, 

 is bent in three places, so that the forebrain extends back 

 and covers the midbrain and hindbrain. Both in man and 

 in the frog the central nervous system is hollow; hence 

 the cavities found within the forebrain are continuous 

 through the midbrain and hindbrain, with a small tube that 

 runs within the spinal cord. 



Section of Forebrain. In a cross section of the cerebral 

 hemispheres one finds on the outside a covering of gray 

 matter known as the cor'tex (Latin cortex = bark). Since 

 this follows all the elevations and depressions on the surface 

 of th.0 brain, it is clear that the convolutions largely increase 

 the amount of gray matter. The interior mass of the brain 

 is formed of white matter, in which are several important 

 masses of gray tissue (ganglia). In the cord, as we have 

 seen, the gray .matter is found in the central regions; in 

 the medulla the gray and white matter are more or less 

 intermingled ; while in the forebrain most of the gray matter 

 is found on the outside. 



Microscopic Structure of the Brain The gray matter of 

 the brain, like that of the spinal cord, consists of countless 

 nerve cells of various shapes and sizes, each with its 

 several rootlike processes and its single axis cylinder 

 process. Some of the fibers in the brain connect the dif- 



