804 TEE NERVES. 



of the spinal cord and brain, while it has no communication with the lymphatics 

 in the vicinity of the nerve. It follows that any increase of pressure in the 

 lymphatic spaces of the central organs, will affect the nerve-ramifications 

 throughout the body.) 



On the track of certain nerves is observed a greyish enlargement, or ganglion. 

 This is composed of a mass of nerve-cells situated on the course of the tubes. 

 All the upper roots of the spinal nerves are provided with a ganglion. In these 

 spinal ganglia in Mammalia, the nerve-cells are unipolar, and the single prolon- 

 gation is directed towards the periphery. Ranvier has demonstrated that this 

 prolongation is united to the nerve-tubes of the roots of the nerves at an 

 annular constriction, and that from this arrangement there result T or Y-shaped 

 tubes. 



Division. — Nerves are divided, with reference to their destination, into two 

 principal groups : 1. The cerebro-sjnnal nerves, or nerves of animal life ; 2. The 

 ganglionic nerves, or nerves of organic life. 



Cerebrospinal nerves. — These emanate directly from the cerebro-spinal axis, 

 and are divided into two secondary groups : 1. The cranial or encephalic nerves, 

 which arise in the brain, and make their exit by the foramina at the base of the 

 cranium, to be distributed almost exclusively in the head. 2. The spinal or 

 rachidian nerves, arising in the spinal cord, and passing to the muscular or 

 tegumentary parts of the trunk and limbs, through the intervertebral foramina. 



After what has been said with regard to the apparatus of innervation, we 

 know that the fibres composing these cords are distinguished — by their point 

 of origin and their properties — into fibres of superior origin or of centripeUd 

 conductibility, and fibres of inferior origin or of centrifugal cmiducfibilifg. The 

 first have a ganglion on their course. 



The cerebro-spinal nerves are exclusively formed of the first description of 

 fibres, and are named sensitive nerves, as they conduct the stimulus which brings 

 into play the sensibility of the brain. They are distinguished as nerves of general 

 sensibility and nerves of special sense. The first are destined to convey all 

 stimuli except those produced by light, sounds, or odoriferous particles ; the 

 second exclusively conduct the latter. 



The nerves which are composed only of fibres of the second kind are called 

 motor nei'ves, because it is they which carry to the muscles the spontaneous 

 stimulus to motion originated by the will. 



Those which are composed at once of motor fibres and fibres of general 

 sensibility, constitute the mixed nerves ; these form the largest category. 



Ganglionic Nerves. — These nerves — collectively representing the great sympa- 

 thetic system — form below and on the sides of the spine, two long cords, rendered 

 moniliform by the presence of ganglionic enlargements. In the constitution of 

 these cords nearly all the cerebro-spinal nerves concur ; their ramifications — 

 frequently ganglionic also — are sent to the viscera of the neck, the thorax, and 

 the abdomen. 



In these nerves of organic life are found the two kinds of nerve-tubes — fibres 

 of centripetal and fibres of centrifugal conductibility. But these tubes appear to 

 have only very indirect relations with the brain, for the will has no influence 

 over the organs which receive their nerves from the great sympathetic ; and 

 besides this, in health the excitations developed in these organs are all reflected by 

 the spinal cord, and do not provoke in any way the special activity of the brain 

 — they are not felt. 



