488 THE NERVOUS SYSTEM. 



When the center is stimulated, arterial pressure is increased on 

 account of the constriction of the vessels. 



Vasomotor Nerves. The vasomotor nerves, which originate in 

 the cells of the vasomotor center in the bulb, pass down the lateral 

 column of the spinal cord, and it is believed that they arborize 

 around the cells of the subsidiary centers in the spinal cord, 

 although the precise location of these centers has not been deter- 

 mined. The cells in these subsidiary centers give rise to axis- 

 cylinders which form a part of medullated nerve-fibers that enter 

 as component parts of the anterior roots of the spinal nerves. 



Vasoconstrictor Nerves. These nerves carry impulses which 

 cause constriction of the arterioles. They pass out from the cord 

 in the anterior roots of the spinal nerves from the second thoracic 

 to the second lumbar, which they leave by the white rami com- 

 municantes, passing into the sympathetic ganglia situated along the 

 vertebral column. These ganglia contain cells around which the 

 nerve-fibers arborize, and they are spoken of as cell stations. From 

 these cells axis-cylinder processes are given off which are continued 

 as non-medullated fibers and which carry the impulses that orig- 

 inate in the vasomotor centers. 



Vasodilator Nerves. The description of the vasoconstrictor 

 fibers just given applies in general to the vasodilator, though there 

 are some marked exceptions, for while, as a rule, they pass out 

 together, still some do not. A striking example of this is the 

 chorda tympani, which is given oif from the facial. Nor do the 

 dilator nerves arborize around the cells of the ganglia of the 

 sympathetic chain, but pass through these and lose their medullary 

 sheaths in the collateral ganglia, such as the semilunar, around 

 whose cells they arborize. 



Depressor Nerve-fibers. Between the heart and the medulla 

 are nerve-fibers which carry impulses from the heart to the vaso- 

 motor nerve-center, which impulses inhibit the center, and thus 

 dimmish the impulses to the muscular coat of the arteries, thereby 

 causing the arteries to dilate and reducing arterial pressure. In 

 the rabbit these fibers are together and form the depressor nerve, 

 but in most animals they are joined with the fibers of the pneumo- 

 gastric. By means of these fibers the nerve-center can be inhibited 

 and arterial pressure lessened, thus reducing the work of the 

 heart. 



Pons Varolii. The pons Varolii (tuber annulare or meso- 

 cephalon) is situated just above the medulla, and is composed 

 of three sets of fibers and of some gray matter. The first set 

 consists of superficial transverse fibers which cross the upper 

 part of the medulla and connect the two hemispheres of the cere- 

 bellum, forming at the sides the crura cerebelli or middle 

 peduncles. The second is made up of longitudinal fibers which 

 come from the pyramids of the medulla and pass on to help form 

 the crura cerebrK The third set is also transverse and is deeply 



