576 THE KERVOTJS SYSTEM. 



time, is governed. When these vaso-motor nerves are 

 intact, the muscular tissue of the arteries is always in a 

 state of tonic contraction, which varies in degree at 

 different times. When they are divided, the muscular 

 fibres in which they are distributed are paralyzed, and the 

 blood-vessels become dilated. The most usual experiment 

 in illustration of 4 these facts is performed by exposing in a 

 rabbit the cervical sympathetic, from which vaso-motor 

 branches are given to the blood-vessels of the head and 

 neck. On dividing the nerve, the blood-vessels of the 

 same side are paralyzed, and the stream of blood, now un- 

 controlled, dilates them. The effect is best seen in the ear, 

 the blood-vessels of which become manifestly larger than 

 those of the opposite side ; while the part becomes redder 

 and warmer from the increased quantity of blood circu- 

 lating through it. On galvanizing the upper divided 

 extremity of the nerve, the muscular fibres of the blood- 

 vessels respond to the stimulus by again contracting, and the 

 parts become paler, colder, and less sensitive than natural. 

 The vaso-motor nerves arise directly from the sympa- 

 thetic. Thus the blood-vessels of the head and neck are 

 supplied by branches from the superior cervical ganglion, 

 those of the thorax from the cervical and upper dorsal 

 ganglia, those of the abdomen chiefly by the splanchnic 

 nerves, and so forth. But it is now generally agreed, from 

 the results of experiments by Ludwig and others, that the 

 principal vaso-motor nerve-centre, with which all these nerves 

 communicate, and by which their action is regulated, is 

 situate in the medulla oblongata or, in other words, that 

 the vaso-motor fibres, arising from this nerve-centre, pass 

 down the spinal cord, and issuing by the anterior roots of 

 the spinal nerves, enter the various ganglia on the preo- 

 vertebral cord of the sympathetic, and thence reach their 

 destination, probably taking with them fibres which arise 

 in the ganglia through which they pass. The vaso-motor 

 centre in the medulla appears to have a regulating power 



