190 PRINCIPLES OF PHYSIOLOGY 



Thus we have many reflex mechanisms, 

 which do not involve higher centres. Sensory 

 fibres pass up the back part of the cord to 

 higher and higher centres, calling forth higher 

 reflex mechanisms, but many, as already 

 indicated, ultimately reach the sensory part 

 of the cerebrum, and give rise to consciousness or 

 sensations of various kinds. The sensory paths 

 are therefore mainly in the posterior part of 

 the cord, and these pass ultimately to the brain 

 on the opposite side, the crossing taking place in 

 the cord and in the medulla. Thus sensory im- 

 pulses from the right side ultimately reach the 

 left cerebral hemisphere, and vice versa. Many 

 sensory impulses also reach the cerebellum, or 

 lower brain. Voluntary motor impulses arise 

 in the cerebrum, pass downwards through the 

 lower parts of the brain, cross to the other side 

 in the bulb or medulla, run down the anterior 

 part of the cord, and, as they pass down, they 

 turn into the grey matter and end by arboriza- 

 tions (like the twigs of a tree) that are close to, 

 but do not touch, the dendrites of large 

 neurones in the grey matter. These give off 

 the axons that become the nerves of the 



