ORIGIN OF NERVES BRAIN. 367 



ganglion near their attachment with the cord. This ganglion is observed 

 upon the posterior roots of all the spinal nerves, and also upon the corre- 

 sponding 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 function with the nerves of sensation, and estab- 

 lishes an analogy with the spinal nerves. 



According to Mr. Grainger, both roots of the spinal nerves, as well as 

 of most of the cerebral, divide into two sets of fibres upon entering the 

 cord, one set being connected with 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 fibres 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, however, be 

 taken 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 anenceph- 

 alous foetuses, where the nerves are beautifully and completely foi me d, 

 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 com- 

 municating it to the muscles, that the latter may instantly remove the arte- 

 ries from impending injury. 



The Sympathetic system consists of numerous ganglia, of communicat- 

 ing branches passing between the ganglia, of others passing between the 

 ganglia and the cerebro-spinal axis, and of branches of distribution which 

 are remarkable for their frequent and plexiform communications. The 

 sympathetic nerves also differ from other nerves in their colour, which is 

 of a grayish pearly tint. 



The capillary vessels of nerves are very minute. They run parallel 

 with the nervous fasciculi, and every here and there are connected by 

 transverse communications, so as to give rise to a net-work composed of 

 oblong meshes very similar to the capillary system of muscles. 



The nervous system may be divided for convenience of description 

 into 1. The brain. 2. The spinal cord. 3. The cranial nerves. 4. The 

 spinal nerves. 5. The sympathetic system. 



THE BRAIN. 



The brain is a collective term which signifies those parts of the ne.-vous 

 system, exclusive of the nerves themselves, which are contained within the 

 cranium ; they are the cerebrum, cerebellum, and medulla oblongata.* 



* The weight of the human brain, according to Scemmering, is 2ft. 5^. to 3ft. l. 7^. 



