452 THE NERVOUS SYSTEM. 



coveries of such ganglia will probably diminish yet further 

 the number of instances in which the involuntary movements 

 appear to be effected independently of central nervous in- 

 fluence. 



Respecting the influence of the sympathetic nerve in nutri- 

 tion and secretion, we may refer to the chapters on those pro- 

 cesses. 



The influence of the sympathetic nerves on the bloodvessels 

 has been already referred to in the section on the Arteries. It 

 was stated that the muscular tissue of the bloodvessels was 

 supplied by sympathetic nerve-branches, called from their dis- 

 tribution and function vaso-motor nerves ; and that by these the 

 condition of the vessels with respect to contraction or relax- 

 ation, and therefore to the stream of blood which flowed through 

 them in a given 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 dif- 

 ferent times. When they are divided, the muscular fibres in 

 which they are distributed are paralyzed, and the bloodvessels 

 become dilated. The most usual experiment in illustration of 

 these facts is performed by exposing in a rabbit the cervical 

 sympathetic, from which vaso-motor branches are given to the 

 bloodvessels of the head and neck. On dividing the nerve, the 

 bloodvessels of the same side are paralyzed, and the stream of 

 blood, now uncontrolled, dilates them. The effect is best seen 

 in the ear, the bloodvessels of which become manifestly larger 

 than those of the opposite side ; while the part becomes redder 

 and warmer from the increased quantity of blood circulating 

 through it. On galvanizing the upper divided extremity of 

 the nerve, the muscular fibres of the bloodvessels 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 sympathetic. 

 Thus the bloodvessels 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 experi- 

 ments 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 ob- 

 longata or, in other words, that the vaso-motor fibres, aris- 

 ing 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 prsevertebral cord of the sympathetic, 

 and thence reach their destination, probably taking with them 



