192 PRINCIPLES OF PHYSIOLOGY 



we have a kind of double nervous mechanism. 

 Thus, when a nervous impulse causes flexors 

 to contract there is, at the same time, an 

 influence which inhibits or restrains the 

 extensors. The nervous machinery therefore 

 is often very complicated. 



108. The part of the cerebro-spinal system 

 within the skull consists of the following struc- 

 tures : (1) a double chain of large masses 

 of grey and white nervous matter, forming 

 from before backwards ; (a) the corpora 

 striata, (b) the optic thalami, and (c) the 

 corpora quadrigemina ; (2) still farther back 

 (a) the pons, (b) the bulb or medulla ; (3) 

 covering the whole of these masses we find the 

 two hemispheres of the cerebrum ; and (4) on 

 the back of the pons and bulb and below the 

 posterior part of the cerebrum, we find the 

 cerebellum. These structures are all connected 

 with each other by strands of white matter 

 formed of nerve fibres, while grey matter is 

 found in masses or nuclei. The fibres 

 carry impulses either upwards or down- 

 wards. There are also numerous fibres 

 passing from one lateral half of the brain to 



