THE CIRCULATION. 197 



2. Vaso-dilator nerves. In addition to the vaso-con- 

 strictor nerves there are vaso-dilator nerves. These nerves 

 are not, however, in a state o-f tonic excitation, but are 

 brought into play at special times only. Thus, when a 

 voluntary muscle is made to exercise, the arteries supplying 

 that muscle at once dilate. This might at first seem due 

 to an inhibition of that part of the constrictor center gov- 

 erning the arteries of these muscles, but there can actually 

 be found nerves which when stimulated cause a direct di- 

 latation. These dilator nerves come from the cerebro- 

 spinal system and are most apparent in the voluntary mus- 

 cles and numerous glands (submaxillary w and parotid) . 

 They run to the intrinsic ganglia distributed through the 

 muscular coat of the arteries (to which also the vaso-con- 

 strictor nerves run) and here inhibit the action of these 

 vaso-constrictor nerves, the result of which is, of course, a 

 dilatation. They run along with the trunks of the cranial 

 and spinal nerves, and anatomically are not distinguishable 

 from them. When the spinal nerve going to the muscle is 

 cut and the peripheral end stimulated not only does the 

 muscle contract, but the artery in the muscle dilates. Such 

 dilator nerves serve as additional means to regulate the 

 local supply of blood, and in the case of muscles, probably 

 accompany the motor nerves so that the best activity of the 

 muscle would be made possible by simultaneous increase in 

 the supply of blood to it. The vaso-dilator center lies in 

 the medulla, also, but seems not to play such an important 

 role as the constrictor center. 



3. Comparison of the innervations of the heart and 

 blood vessels. There is a striking homofogy between the 

 nerves of the heart and the blood vessels. 



1. Both have distributed in their muscles intrinsic 

 ganglia. 



2. To both there run nerves, the stimulation of which 

 causes an increase in the contraction. In the case of the 

 heart this nerve is called the cardio-accelerator, and causes 



