NERVOUS SYSTEM OF VERTEBRATA. 269 



protuberances from the Medulla Oblongata. Moreover, the fact that animals 

 are capable of living- without the brain, whilst they at once die if deprived of 

 the spinal cord, sufficiently demonstrates this. The spinal cord, then, when 

 viewed in relation to the nervous system of the Invertebrata, may be regarded 

 as including their respiratory, stomato-gastric, and pedal ganglia. That these 

 should be associated together, can scarcely be considered remarkable. It is 

 obviously convenient that they should all be inclosed in the bony sheath pro- 

 vided for their protection ; and their closer relation favors that sympathy of 

 action, which is so important in animals of such complex structure and 

 mutually dependent functions, as the higher Vertebrata. An animal either 

 congenially or experimentally deprived of its cerebral hemispheres, is very 

 much in the condition of one of the Acephalous Mollusca. It can perform 

 those respiratory movements, on which depend the maintenance of its circu- 

 lation, and consequently its whole organic life; it can swallow food brought 

 within its reach, and it can, in some degree, exert its locomotive powers to 

 obtain it; but it is unconscious of the direction in which these can be best 

 employed, and is dependent upon the supplies of food that come within its 

 grasp. The Acephalous Mollusca are so organized, that they find support 

 from the particles brought in by their respiratory current; but the more 

 highly-organized Vertebrata are not capable of so existing, and they must 

 have their food provided for them by an exertion of the mental powers. So 

 long as an anencephalous Vertebrated animal is duly supplied with its requi- 

 site food, so long may it continue to exist, although in a state analogous to 

 that of profound sleep; and thus it is seen, that the operations of the Brain 

 are not immediately connected with the maintenance of the organic functions ; 

 the movements requisite for these being carried on, as in the lower animals, 

 through the instrumentality of ganglionic centres and nerves specially appro- 

 priated to them. 



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

 and efferent fibres of the nerves, has been satisfactorily determined. The 

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

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

 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, before any other physiolo- 

 gist came into the field.* 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 main- 

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

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

 in the reception of sensations 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 posterior 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 posterior especially sensory. But the recently attained knowledge of 

 the reflex function of the spinal cord, enables the latter portion of these phe- 

 nomena to be easily explained. The motions excited by irritating the pos- 

 terior root are entirely dependent upon its 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 excita- 



' See British and Foreign Medical Review. Vol. ix., p. 140, &c. 



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