632 MANUAL OF PHYSIOLOGY. 



the centre may be such that these remoter influences may become 

 quite powerless. This uncontrollable condition of the centre is 

 established when the blood flowing through it is abnormally 

 venous and the cells become over-stimulated. We all know how 

 short a time we can hold our breath by voluntary checking of the 

 centre, and most people have had occasion to observe the inordi- 

 nate and painful efforts of a person whose respiration is interfered 

 with by disease. When the dyspnoea becomes intense, nearly all 

 the muscles in the body are called into action. Thus, in quiet 

 breathing comparatively few nerve cells in the medulla carry on 

 the work of respiration, but under certain emergencies they can 

 call to their aid the entire motor areas of the gray substance of 

 the spinal cord, and thus give rise to a kind of general effort. 

 Hence we often hear of a convulsive centre in the medulla being 

 placed in close relation to the respiratory centre. In cases, 

 namely, irritation of the air-passages, or imperfect oxidation of 

 the blood, the convulsive centre comes under the command of the 

 cells of the respiratory centre, which can then excite coughing, 

 sneezing, or convulsive inspiratory effort. 



As already mentioned, the convulsion of asphyxia may also, in 

 part at least, be explained by the impure blood acting as a stim- 

 ulus to the cells of the cord itself. 



THE VASO-MOTOR CENTRE. 



It has already been stated that groups of cells exist in the gray 

 part of the spinal cord, which, according to the class of animal, 

 have more or less direct influence upon the muscles in the coats 

 of the vessels. Thus in a frog, whose brain and medulla have 

 been destroyed, in some hours the vessels of the web regain a 

 considerable degree of constriction, which is again lost if the cord 

 be destroyed. In the dog the vessels of the hinder limb also 

 recover their tone more or less perfectly in a few days after the 

 spinal marrow has been cut in the dorsal region, although just 

 after the section they are widely dilated from the paralysis of 

 their muscular coats. In a few days, then, the cells of the cord 

 can learn to accomplish, of their own accord, work which they 



