700 THE NERVES. 



THIRD SECTION. 

 THE NERVES. 



THE nerves represent the peripheral portions of the nervous system, and 

 are cords ramifying in every part of the body, having their origin in the 

 medullary axis or its encephalic prolongation. Before commencing thciv 

 special study, it is necessary to possess a summary notion of the principal 

 distinctions of which they are susceptible, with regard to their origin, 

 distribution and termination. 



STRUCTURE. The nerves are formed by an aggregation of the nerve-tubes 

 already described. These are grouped in primary fasciculi, which are 

 rectilinear or slightly undulating, and enveloped in a sheath of delicate con- 

 nective tissue the perineurium. These primary fasciculi are again assem- 

 bled in bundles to compose secondary fasciculi, which are maintained by a 

 layer of fibrous connective tissue thicker than the perineurium. Finally, 

 these secondary bundles by their union constitute the nerve, around which 

 the connective tissue becomes condensed, and found the neurilemma. 



Vessels traverse the connective tissue separating the fasciculi from each 

 other ; they anastomose in a network whose elongated meshes are parallel 

 with the nerve-tubes, and they are also surrounded by the nervi nervorum. 



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

 ganglion. This is composed of a mass of nerve-cells, which are generally 

 bipolar, and are situated on the course of the tubes. It is not quite known 

 whether some of these tubes are not merely placed alongside the ganglion. 



DIVISION. Nerves are divided, with reference to their destination, 

 into two principal groups: 1,* The cerebro-spinal or nerves of animal life; 

 2, The ganglionic or nerves of organic life. 



CEREBRO-SPINAL NERVES. These emanate directly from the cerebro- 

 spinal axis, and are divided into two secondary groups: 1, The cranial 

 or encephalic nerves, which originate in the encephalon, 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 in regard to the apparatus of innervation, we 

 know that the fibre 3 composing these cords are distinguished, by their point 

 of origin and their properties, into fibres of superior origin or of centripetal 

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

 first have a ganglion on their course. 



The cerjbro-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 sensations (or sense). The 

 first are destined to convey all stimuli except those determined 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 nerves, 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. 



