THE BRAIN. 505 
pora striata and optic thalami), the crura cerebri, corpora quad- 
rigemina, pons Varolii, and medulla oblongata, and especially 
if the section be unilateral. We have already seen that several 
of these parts are concerned in muscular co-ordination; hence 
the disorderly character of any movements that might now re- 
sult when any part of this related mechanism is thrown out of 
gear, so to speak; but, apart from that, we think that the view 
presented in the previous sections is applicable in this case 
also, while the forced movements themselves throw light upon 
the symptoms following injury to the semicircular canals. 
When that constant afflux of sensory impulses toward the 
nervous centers is interfered with, as must be the case in such 
sections as are now referred to, it'is plain that the balance in 
consciousness must be disturbed; confusion results, and it is 
not surprising that, instead of a passive condition, one marked 
by disorderly movements should result in an animal, since 
movement so largely enters into its life-habits. It is important 
to remember, in this connection, that the great highway of im- 
pulses between the cerebral cortex and other parts of the brain 
and the spinal cord lies in the very parts of the encephalon we 
are now considering. o 
FUNCTIONS OF THE CEREBRAL CONVOLUTIONS, 
Comparative.—It will conduce to the comprehension of this 
subject if some reference be now made to the development of 
the brain in the different groups of the animal kingdom. 
Invertebrates not only have no cerebrum, but no brain in 
the strict sense of the term as applied to the higher mammals. 
In most forms of this great subdivision of the animal kingdom, 
the first or head segment is provided with ganglia arranged in 
the form of a collar around the cesophagus, by means of com- 
missural nerve connections; so that the nervous supply of the 
head is not widely different from that of the other segments 
of the body. But as we ascend in the scale among the in- 
vertebrates these ganglia become more crowded together, and 
so resemble the vertebrate brain with its massed ganglia and 
numerous connections through nerve- fibers, etc. But in this 
respect we find great difference among vertebrates. We can 
recognize, on passing upward from the Amphioxus, destitute 
of a brain proper, to man, all gradations in the form, relative 
size, multiplicity of connecting ties, etc. 
Speaking generally, there is great difference in the weight 
