88 SYMPATHETIC NERVES. 



center (probably in the spinal cord) ; and by reflex action the 

 arteries supplying the lower limbs are widened, and these 

 muscles receive more blood. But these muscles make up a 

 very considerable part of the weight and bulk of the body. 

 While in action they take the lion's share of the blood. The 

 brain, at such a time, would receive less ; and 

 it would be folly to expect the brain to work 

 at its full capacity while the blood was called 

 away to other organs. 



When we exercise vigorously the heart 

 beats faster, and this of itself 

 would tend to increase the 



Sympathetic, 



blood supply to all organs. But Ganglia 

 this mechanism of widening the 

 channel leading to the working organs, while 

 the arteries to the other organs are made 

 smaller, or at least are not enlarged, solves 

 the problem of supplying each part accord- 

 ing to a greatly varying need, while not 

 sending too much to a part not needing it. 



When we sit quietly intent upon study, 

 the brain has a larger supply of blood, and 

 other parts now have less ; in such condition 

 the feet may easily grow cold. 



If we wish to draw away some of the 

 blood from the brain after prolonged study, 

 we take a little gentle exercise to better pre- 

 pare for Sleep. Fig. 31. Ventral View 



Dissection of the Sympathetic Ner- f s P inal Cord wiih 



Sympathetic Ganglia 



vous System. In a cat or a rabbit open of one side. 

 the chest cavity, and pull the heart and 

 lungs to one side. Close to the spinal column there maj 7 be 

 seen a white nerve with a ganglion for every rib. This is the 



