THE BRAIN. 485 



direction of the movements, this is not so in all cases ; nor does 

 it, in itself, explain the fact of their being plainly not volun- 

 tary in the usual sense. 



The parts of the brain, which, when injured, are most liable 

 to be followed by forced movements are the basal ganglia (cor- 

 pora striata and optic thalami), the crura cerebri, corpora quad- 

 rigemina, pons Varolii, and medidla 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 af9.ux 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 conscious- 

 ness must be disturbed ; confusion results, and it is not sur- 

 prising that, instead of a passive condition, one marked by dis- 

 orderly movements should result in an animal, since movement 

 so largely enters into its life-habits. It is important to remem- 

 ber, in this connection, that the great highway of impulses 

 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. 



FUNCTIONS OP 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 oesophagus, 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 



