GENERAL SUMMARY. 651 



687. Having thus considered the principal attributes of the ganglionic centres 

 of the Cerebro-Spinal system, we have next to inquire into those of the nerve- 

 trunks which are connected with them. It is only in the Vertebrata that the 

 difference between the afferent and efferent fibres of the nerves has been satis- 

 factorily 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 were not 

 satisfactory, he virtually determined the respective functions of their two roots, 

 by experiments and pathological observations upon the cranial nerves, some of 

 which contain only one class of fibres to the exclusion of the other, before any 

 other physiologist came into the field. 1 Subsequently his general views were 

 confirmed by the very decided experiments of Miiller; but until very recently, 

 some obscurity hung over a portion of the phenomena. It was from the first 

 maintained by Magendie, and has been subsequently asserted by other physi- 

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

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

 when the anterior roots were touched, the animal gave signs of pain, at the same 

 time that convulsive movements were performed : and that, on touching the pos- 

 terior roots, not only the sensibility of the animal seemed to be affected, but 

 muscular motions were excited. These physiologists were not willing, therefore, 

 to admit more than that the anterior roots were especially motor, and the pos- 

 terior especially sensory. But the knowledge we now possess of the reflex 

 function of the Spinal Cord enables the latter 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 motor impulse 

 in the Spinal Cord, which is propagated through the 'anterior roots to the peri- 

 phery of the system. The converse phenomenon, the apparent sensibility of 

 the anterior roots, has been explained by the experiments of Dr. Kroneuberg, 2 

 which seem to prove that it is dependent upon a branch from the posterior 

 roots passing into the anterior roots at their point of inosculation, and then 

 directing itself towards the cord ( 692). 



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

 the central organ, in which it loses itself at one extremity, and the muscle or 

 organ of sense in which it terminates at the other. Each nervous trunk is 

 made up of several fasciculi of these fibres ; and each fasciculus is composed of 

 a large number of the ultimate fibres themselves. Although the fasciculi occa- 

 sionally intermix and exchange fibres with one another (as occurs in what is 

 termed a plexus), the fibres themselves never inosculate. Each fibre would 

 seem, therefore, to have its appropriate office, which it cannot share with another. 

 Several objects appear to be attained by the plexiform* arrangement. In some 

 instances it serves to intermix fibres, which have endowments fundamentally 

 different; for example, the Spinal Accessory nerve, at its origin, appears to be 

 exclusively motor, and the roots of the Pneumogastric to be as exclusively 

 afferent ; but by the early admixture of these, a large number of motor fibres 

 are imparted to the Pneumogastric, and are distributed in variable proportion, 

 with its different branches ; whilst few of its sensory filaments seem to enter 

 the Spinal Accessory. In other instances, the object of a plexus appears to be 



1 See "Brit, and Foreign Med. Review," vol. ix. p. 140, &c. 



2 "Muller's Archiv.," 1839, Heft v.; and "Brit, and For. Med. Rev.," vol. ix. p. 547. 



