692 A MANUAL OF PHYSIOLOGY 



thalamus. Such fibres pass from the thalamus to the frontal 

 and parietal regions through the anterior border of the 

 internal capsule in front of the frontal fibres previously de- 

 scribed as running in the anterior limb of the capsule to the 

 pons; and from the thalamus to the occipital region through 

 the extreme posterior border of the internal capsule, behind 

 the occipital fibres that proceed to the pons. The fibres 

 that connect the thalamus with the occipital cortex are 

 spoken of as the optic radiation. Some of the fibres of the 

 optic radiation, however, proceed, not to the thalamus, but 

 to the anterior corpus quadrigeminum and the lateral geni- 

 culate 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. 685). 



We have purposely omitted to enumerate many 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 hemisphere are in communication by 

 short connecting loops of 'association ' fibres (Fig. 251), it 

 will be seen that the linkage of the various parts of the 

 central nervous system is extremely complex ; that an excita- 

 tion, blocked out from one path, may have the choice of 

 many alternative routes ; and that the apparent 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 once it 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 

 collaterals. 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 substance, if such a substance there be, which 



