ORIGIN OF NERVES. 403 



of the spinal cord, and possesses 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 dissec- 

 tion 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 was 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 he thence named the 

 respiratory tract. The microscope has succeeded in making no struc- 

 tural distinction between the anterior and posterior roots of the spinal 

 nerves ; but the latter are remarkable from possessing a ganglion 

 near 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 con- 

 sidered 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 researches of Mr. Grainger have made an important addition 

 to our knowledge of the mode of connection 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 filaments upon 

 entering the cord, one set being connected to the grey 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 

 grey 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 connection of a nerve with the cerebro-spinal axis is called, for 

 convenience of description, its origin : this term must not, however, 

 be received literally, for each nerve is developed in the precise 

 situation which it occupies in the body, and with the same relations 

 that it possesses in after life. Indeed, we not unfrequently meet 

 with instances in anencephalous foetuses where the nerves are beau- 

 tifully and completely formed, while the brain and spinal cord are 

 wanting. The word " origin" must therefore be considered as a relict 

 of the darkness of preceding ages, when the cerebro-spinal axis was 

 looked upon as the tree from which the nerves pushed forth as 

 branches. In their distribution the spinal nerves for the most part 

 follow the course of the arteries, particularly in the limbs, where they 

 lie almost constantly to the outer side and superficially to the vessels, 

 as if for the purpose of receiving the first intimation of danger and of 



