i 9 2 PHYSIOLOGY FOR BEGINNERS CHAP. 



many other ways a very important part of the brain. We 

 have seen that the respiratory movements of the chest are not 

 only regulated by, but originated by nervous impulses arising 

 in a part of the bulb called the respiratory centre, and injury 

 to the bulb causes respiration to stop, and so produces death. 

 We have also seen that the beat of the heart is regulated by 

 nervous impulses sent from the bulb, and that the size of the 

 small arteries is regulated by impulses sent along the vaso- 

 motor nerves from a part of the spinal bulb called the vaso- 

 motor centre. There are, in addition to these, other parts of 

 the bulb which govern the act of swallowing, the secretion of 

 saliva, and other processes. In addition to being the seat 

 of these important regulating functions, the bulb forms the 

 path through which all impulses from the cerebral hemi- 

 spheres and other parts of the brain must pass on their way 

 to the spinal cord and spinal nerves, and similarly the path 

 for all impulses from the spinal nerves and spinal cord to the 

 cerebral hemispheres. It is a very remarkable fact that the 

 nerve fibres which carry motor impulses descending from the 

 brain to the spinal cord cross over rather suddenly from one 

 side to the other on their way through the spinal bulb. So 

 that the fibres which carry motor impulses from the right 

 cerebral hemisphere, having passed downwards in the crus 

 cerebri on the right side to the bulb, cross over in the bulb 

 to the left side and then continue downwards in the white 

 matter of the left side of the cord. Certain injuries to one 

 cerebral hemisphere, for instance, the right, such as the burst- 

 ing of a blood-vessel in it, cause paralysis of the opposite side 

 of the body, namely, in this case, of the left arm, left leg, and 

 left side of the face and trunk. 



In a similar way sensory impulses received from one side, for 

 instance, the left side of the body, are carried by fibres in the 

 white matter of the same, the left side of the spinal cord, which 

 cross over partly in the spinal cord itself, partly in the bulb, 

 to the other, the right side, and so reach the opposite, the 

 right cerebral hemisphere, where they give rise to definite 

 sensations. So it is that after certain injuries to the right 

 cerebral hemisphere impulses starting from the skin on the 

 left side of the body do not give rise to sensations in the brain. 

 The injury to one of the hemispheres causes both paralysis, 



