THE NERVOUS SYSTEM. 99 



(see Fig. 55 and Fig. 57); they are reckoned numerically 

 from before backwards. The first twelve pairs arise within 

 the cranium, and leave the cavity by apertures in the 

 cranial bones ; the thirty-one pairs which follow arise from 

 the spinal marrow, and leave the osseous canal by open- 

 ings called inter vertebral, as being placed between the 

 vertebrae. 



Each of these pairs of nerves is formed 

 of a great number of filaments, enclosed in 

 a neurilemma. These elementary fibres are 

 extremely fine ; they do not unite with each 

 other, but pass from the central extremity 

 of the nerve to the peripheral. At their 

 origin, the fibres composing the nerve are 

 called roots, and are grouped in the spinal 

 nerves into anterior and posterior roots. 



On the posterior root is a ganglion through 

 which the filaments pass, and then unite with 

 the anterior root, On certain of the cranial nerves, some of 

 the roots also have ganglions. Experiment has shown that 

 the posterior roots are nerves of sensation ; the anterior, 

 nerves of motion : when nerves unite with each other by 

 what is called anastomosisf (although they do not really 

 unite, but merely exchange fibres), there results what is 

 called a plexus.^ Finally, in the various organs the nerves 

 seem to terminate by forming wide plexuses or loops. 



192. Ganglionary System. This system which has 

 also been called the nervous system of organic life, the sys- 



of the neck and thorax belonging to the sympathetic system of nerves. 

 1, pneumogastric nerve, the principal branches of which form plexuses with 

 the great sympathetic, to supply the lungs, heart, stomach, &c. ; 6, 7, branches 

 of the pneumogastric, supplying the larynx ; 9, 9, recurrent nerve, a branch 

 of the pneumogastric ascending from the base of the neck to the larynx ; 

 10, 11, cardiac branches proceeding to the heart ; 13, pulmonary plexus ; 

 14, lingual nerve ; 15, terminal portion of the great hypoglossal ; 16, glosso- 

 pharyngeal nerve ; 17, spinal accessory of Willis ; 18, cervical nerve of the 

 second pair of spinal nerves ; 19, third cervical pair ; 23, 26, 27, 28, pairs of 

 cervical nerves, unitint: with the first dorsal to form the brachial plexus; 

 24, superior cervical ganglion of the great sympathetic ; 25, middle cervical 

 ganglion ; 26, inferior cervical ganglion ; 27 to 30, dorsal ganglions. 



* A portion of the spinal marrow. a, medulla spinalis ; b, posterior root 

 of one of the nerves ; c, ganglion situated on this root ; d, anterior root of 

 the same nerve, about to unite with the posterior root after this latter has 

 passed the ganglion; e, common trunk formed by both roots; f, small 

 branch proceeding to anastomose with the great sympathetic. 



t It is not then a true anastomosis. 



J Plexus, from plecto (I intermingle), is a term applied also to the blood- 

 vessels, and means merely a sort of network. 



H2 



