360 CLASSIFICATION OF NERVES. 



numerous communications, the sympathetic system, or system of 

 organic life. 



The division of nerves into cranial and spinal is purely arbitrary, 

 and depends upon the circumstance of the former passing through 

 the foramina of the cranium, and the latter through those of the 

 vertebral column. With respect to origin, all the cranial nerves, 

 with, the exception of the first, olfactory, proceed from the spinal 

 cord, or from its immediate continuation into the brain. The spinal 

 nerves arise by two roots ; anterior, which proceeds from the 

 anterior segment of the spinal cord, and possess a motor function ; 

 and posterior, which is connected with the posterior segment, and 

 bestows the faculty of sensation. The motor nerves of the cranium 

 are shown by dissection to be continuous with the motor portion of 

 the cord, and form one system with the motor roots of the spinal 

 cord ; while the nerves of sensation, always excepting the olfactory, 

 are in like manner traced to the posterior segment of the cord, and 

 form part of the system of sensation. To these two systems a third 

 has been added by Sir Charles Bell, the respiratory system, 

 which consists of nerves associated in the function of respiration, 

 and arising from the side of the upper part of the spinal cord in 

 one continuous line, which was thence named, by that distinguished 

 physiologist, the respiratory tract. The microscope has succeeded 

 in making no structural distinction between the anterior and pos- 

 terior roots of the spinal nerves ; but the latter are remarkable from 

 possessing a ganglion near to their attachment with the cord. This 

 ganglion is observed upon the posterior roots of all the spinal nerves, 

 and also upon the corresponding root of the fifth cranial nerve, 

 which is thence considered a spinal cranial nerve. Upon others of 

 the cranial nerves a ganglion is found, which associates them in 

 their function with the nerves of sensation, and establishes an 

 analogy with the spinal nerves. 



The recent researches of Mr. Grainger have made an important 

 addition to our knowledge of the mode of connexion of the nerves 

 with the spinal cord ; he has shown that both roots of the spinal 

 nerves, as well as most of the cerebral, divide into two sets of fila- 

 ments upon entering the cord, one sef being connected to the gray 

 substance, while the other is continuous with the white or fibrous 

 part of the cord. The former he considers to be the agents of the 

 excito-motory system of Dr. Marshall Hall ; and the latter, the 

 communication with the brain and the medium for the transmission 

 of sensation and volition. He has not been able to trace the fibres 

 which enter the gray substance to their termination ; but he thinks 

 it probable that the ultimate filaments of the posterior root join those 

 of the anterior root ; or in the words of Dr. Marshall Hall's system, 

 that the incident fibres (sensitive) are continuous with the reflex 

 (motor). 



The connexion of a nerve with the cerebro-spinal axis is called, 

 for convenience of description, its origin : this term must not, how- 





