CEREBRO-SPINAL NERVE-TRUNKS IN GEKERAL. 571 



the cerebro-spiual system, the lachrymal and sublingual glands receiving 

 large branches from the fifth pair, and the mammary glands from the in- 

 tercostal nerves. 



4(51. Cerebro-Spinai Nerve-Trunks. Having thus considered the principal 

 attributes of the gangliouic centres of the Cerebro-spinal system, we have 

 next to inquire into those of the nerve-trunks which are connected with 

 thorn. It is only in the Vertebrata, that the difference between the afferent 

 and ctf'firnt fibres of the nerves has been satisfactorily determined. The 

 merit of this discovery is almost entirely due to Sir C. Bell, who was led to 

 it by a chain of reasoning of a highly philosophical character ; and although 

 his first experiments on the Spinal nerves w r ere not satisfactory, he virtually 

 determined the respective functions of their two roots, the posterior as 

 wyjw/v/ (afferent), the anterior motor (efferent), by experiments, and by 

 pathological observations upon the Cranial nerves, some of which contain 

 only one class of fibres to the exclusion of the other, before any other physi- 

 ologist came into the field. 1 Subsequently his general views were confirmed 

 by the very decided experiments of Miiller ; but until very recently, some ob- 

 scurity hung over a portion of the phenomena. It was from the first main- 

 tained by Magendie, and has been subsequently asserted by other physiolo- 

 gists, that the posterior and anterior roots of the nerves were both concerned 

 in the reception of impressions and in the production of motions ; for that, 

 on touching the posterior roots, not only the sensibility of the animal seemed 

 to be affected, but muscular motions were excited ; and that, when the anterior 

 roots were touched, the animal gave signs of pain, at the same time that con- 

 vulsive movements were performed. These physiologists were not willing, 

 therefore, to admit inore than that the posterior roots were especially sensory, 

 and the anterior especially motor. But the knowledge we now possess of the 

 "reflex" function of the Spinal Cord enables the former portion of these 

 phenomena to be easily explained. The motions excited by irritating the 

 posterior roots are found to be entirely dependent upon their connection 

 with the Spinal Cord, and upon the integrity of the anterior roots and of the 

 trunks into which they enter ; whilst they are not checked by the separation 

 of the posterior roots from the peripheral portion of the trunk : it is evident, 

 therefore, that excitation of the posterior roots does not act immediately 

 upon the muscles, through the trunk of the nerve which they contribute to 

 form ; but that it excites a reflex motor impulse in the Spinal Cord, which 

 is propagated through the anterior roots to the periphery of the system. 

 The converse phenomenon of the apparent sensibility of the anterior roots 

 of the Spinal Nerves has been recently investigated and satisfactorily ex- 

 plained by M. Browii-Seqnard. If these roots be irritated, the animal 

 usually gives signs of uneasiness ; but if they be divided, and the cut ends 

 nearest the centre be irritated, none such are exhibited: whilst they are still 

 shown when the farther ends are irritated, but not if tha posterior roots are 

 divided. According to M. Browu-Sequard, 2 these phenomena are simply 

 clue to the circumstance, that on irritating the distal extremities of the an- 

 terior roots the muscles supplied by them are thrown into a state of cramp, 

 and pain is experienced from the violent compression of the extremities of 

 the sensory nerves, which of course is no longer felt when the roots of the 

 latter are divided. 



462. Every fibre, there is reason to believe, runs a distinct course, between 

 the central organ, with one of the cells of which it is connected at one ex- 

 tremity, and the organ of sense, muscle, or other tissue, in which it either 



1 See British and Foreign Med. Review, vol. ix, p. 140, etc. 



2 Lectures on the Physiology of the Central Nervous System, 1860, Lecture i. 



