THE CENTRAL NERVOUS SYSTEM 785 



with the occipital cortex are spoken of as the optic radiation. 

 Some of the fibres of the optic radiation, however, proceed, not 

 from the thalamus, but from the anterior corpus quadrigeminum 

 and the lateral geniculate body. The thalamus is also connected 

 with the cortex of the temporal lobe, with the cerebellum, and 

 through the fillet with the posterior part of the tegmental 

 system, the medulla oblongata and the spinal cord (p. 772). 

 Fibres also pass from the inner and deeper part of the thalamus 

 to the lenticular nucleus of the corpus striatum. The thalamus 

 must be regarded as a great sensory centre through which 

 afferent impulses stream on their way to all parts of the 

 cortex. 



We have purposely omitted to enumerate other paths by 

 which the various tracts of grey matter in the brain are brought 

 into communication with each other, and our knowledge of 

 such connections is constantly augmenting. When we add 

 that not only are the cerebral hemispheres united by many 

 ties to the subordinate portions of the cerebro-spinal axis and 

 to each other, but that cortical areas of one and the same hemi- 

 sphere are in communication by short connecting loops of 

 ' association ' fibres (Fig. 334), it will be seen that the linkage 

 of the various parts of the central nervous system is extremely 

 complex ; that an excitation, blocked out from one path, may 

 have the choice of many alternative routes ; and that the ap- 

 parent simplicity and isolation of the pyramidal tracts must not 

 be allowed too far to govern our views of the possibilities open to 

 a nervous impulse onceUt has been set going in the labyrinth 

 of the nervous network. Nor is it only by the main channel 

 of the axis-cylinder that nervous impulses can be conducted. 

 It cannot be doubted that they can also pass along the col- 

 laterals. And the actual route taken by a given impulse is, 

 in all probability, determined not only by anatomical relations, 

 but also by molecular conditions, particularly in the terminal 

 fibrils of the axons, collaterals and dendrites, and in the sub- 

 stance, if such a substance there be, which intervenes between 

 the end arborizations of a neuron and the dendrites or cell- 

 bodies of the neurons with which they lie in contact. So that 

 a road open at one moment may be closed at another. We may 

 suppose that the greater the number of connections between 

 the cells of the central nervous system, the greater is the com- 

 plexity of the processes which may be carried on within it. 

 And, indeed, comparison of the brains of different animals shows 

 that it is not so much by excess in the number of nerve-cells 

 as by the increased complexity of linkage, that a highly-de- 

 veloped brain differs from a brain of lower type ; the higher the 

 brain, the more richly branched are the dendrites and the 



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