500 THE NERVOUS SYSTEM. 



or its superior cervical ganglion extirpated, the pupil on the correspond- 

 ing side becomes contracted from paralysis of its radiating fibres. In 

 quadrupeds, the eyeball is also drawn backward into the orbit, causing 

 partial closure of the upper and lower eyelids, and advance of the third 

 eyelid or " nictitating membrane " over the cornea. This recession of 

 the eyeball is due to paralysis of a muscle composed of unstriped fibres, 

 which exists in most quadrupeds under the name of the " orbital 

 muscle," and which normally maintains the eyeball in a moderate 

 state of protrusion. After its paralysis, the straight muscles of the 

 orbit, being no longer antagonized, produce permanent retraction of 

 the eyeball, and consequently partial closure of the lids. Both the 

 closure of the lids and the narrowing of the pupil are therefore sec- 

 ondary effects of division of the sympathetic nerve. 



But if, under these circumstances, the upper extremity of the divided 

 nerve be stimulated by faradization, the conditions are reversed. The 

 eyeball advances to its former place in the orbit, and the pupil dilates. 

 Both these effects correspond iu degree with the stimulation of the 

 nerve. If the electric current be of moderate strength, the eye may 

 be simply restored for the time to its normal condition. If of greater 

 intensity, it may cause protrusion of the eyeball and enlargement of 

 the pupil to its maximum diameter. When faradization is sus- 

 pended, the pupil again diminishes, and the eyeball returns to its 

 retracted position. 



It is evident, accordingly, that the muscular apparatus of the eye is 

 under the control of two nervous influences, derived respectively from 

 the cerebro-spinal and the sympathetic system. The iris receives all 

 its motor fibres from the ophthalmic ganglion. But those causing con- 

 traction of the pupil come through this ganglion from the oculomo- 

 torius nerve : those causing dilatation are derived, through the same 

 channel, from the central ganglia of the sympathetic system. 



Vasomotor Nerves and Nerve Centres. 



The most important general function of the sympathetic nerves and 

 nerve centres is connected with the blood-vessels and the circulation in 

 different regions of the body. Their filaments and plexuses are espe- 

 cially associated with the arterial branches, which they follow in their 

 subsequent ramification ; and their terminal fibres are largely distributed 

 to the muscular coat of these vessels. Under their influence the muscu- 

 lar elements contract, thus approximating the walls of the artery, and 

 diminishing its calibre. The nerves which excite in this way the con- 

 traction of the blood-vessels are called "Vasomotor nerves," and the 

 nerve centres from which they emanate " vasomotor centres." 



Muscularity and Contractility of the Blood-vessels. So far as their 

 structure is concerned, the arteries are, in great measure, muscular 

 organs. Their middle coat, at least in those of medium and smaller 

 size, contains unstriped muscular fibres mingled with elastic tissue ; 

 and the relative abundance of these fibres increases as the size of the 



