194 PRINCIPLES OF PHYSIOLOGY 



that both sensory and motor, especially 

 motor, tracts cross in the bulb. This part 

 of the brain is therefore all important to life. 

 If it is destroyed the respiratory, cardiac, 

 and vascular mechanisms quickly cease. It is 

 also the seat of reflexes of a complicated 

 character, such as those of swallowing. 



110. The bulb may also be regarded as 

 the most posterior part of the brain, or 

 as the portion of the cord within the 

 skull. It is in a sense one of the most 

 important centres of the body because, 

 as already mentioned, it contains centres 

 for mechanisms absolutely essential to life. 

 A study of its functions also illustrates 

 several fundamental principles. Although not 

 of large dimensions it contains, in addition to 

 fibres passing through it, and conveying 

 impulses upwards and downwards, the follow- 

 ing centres : (a) respiratory centres ; (b) 

 cardiac or heart centres ; (c) vaso-motor 

 centres for the peripheral blood vessels ; 

 and (d) centres for swallowing. As each 

 centre is double, owing to the bilateral sym- 

 metry of the nervous system, there are at 



