270 



FUNCTIONS OF THE NERVOUS SYSTEM. 



Fis. 132. 



lion of the posterior root 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 periphery of the system. The converse phenomenon, the appa- 

 rent sensibility of the anterior roots, has been still more recently explained 

 by the experiments of Dr. Kron'enberg;* which seem to prove, that it is de- 

 pendent upon a branch of the posterior root passing into the anterior root at 

 their point of inosculation, and then directing itself towards the cord ( 304). 

 345. On the other hand, the distinctness of the system of nerves concerned 

 in the simply-reflex actions, from those which minister to sensation, emotion, 

 and volition by their connection with the brain, is by no means so obvious as 

 in the Invertebrated classes. When first pointed out by Dr. Marshall Hall, 

 Avho had grounded his opinion more upon physiological phenomena than 

 upon anatomical facts, the statement did not command general assent; since, 

 Avhile the phenomena were admitted, the inferences which he drew from them 

 were not regarded as necessary results. When, however, the anatomy of 

 the Nervous centres in Vertebrata was more closely inquired into (by Mr. 

 Grainger, who had been partly anticipated by Bellingeri), it was found to 

 present certain phenomena which might be regarded as supporting Dr. M. 

 Hall's views; and when the inquiry was extended to the Invertebrated classes, 

 the confirmation was found to be still more decisive. In our previous sketch 

 these doctrines have been treated as established; since they have been found 

 not only to correspond with the facts disclosed by anatomical research, but to 

 be required by them. We shall now apply them to the nervous apparatus of 

 the Vertebrata. 



346. The Spinal Cord consists of two lateral halves ; 

 these are partially separated, in the higher classes, by 

 the superficial anterior and posterior fissures ; and in 

 Fishes by an internal canal, which is continuous with the 

 fourth ventricle.! This canal is evidently the indication 

 of that complete separation of the two columns, which 

 exists in the lower Articulata; and the fourth ventricle, 

 which in many Fishes remains unclosed (the cerebellum 

 not being sufficiently developed to overlap it), corre- 

 sponds with the passage between the cords uniting the 

 cephalic ganglia, with the first sub-oesophageal, through 

 which the oesophagus passes in all the Invertebrata. 

 The two lateral halves have little connection with each 

 other in Fishes, and the pyramidal bodies at their apex 

 scarcely decussate ; but in ascending toward'^ the higher 

 classes, the communication between the two sides is more 

 intimate, and a larger proportion 6f the pyramidal fibres, 

 crosses to the opposite side. In all the Vertebrata, the 

 true Spinal Cord contains grey substance, or something 

 equivalent to it; thus possessing the character of a con- 

 tinuous ganglion. The proportion of the vertebral column 

 which this ganglionic portion occupies, is, however, ex- 

 tremely variable ; depending principally on the position 

 of the chief organs of locomotion. Thus, in the Eel, 

 and other Vermiform Fishes, it is continued through 

 the whole spinal canal ; whilst in the Lophius and 

 Tetraodon, whose body is less prolonged, and more 

 dependent for its movements upon the anterior extremi- 



* Miiller's Archiv., 1S39, Heft v.: and Brit, and For. Med. Rev., vol. ix. p. 547. 

 t Tins canal may be traced in the Spinal Cord of Man and other Mammalia; but it is 

 nearly obliterated. 



\i-rvous centres in 

 Frog; A, olfactive ganglia; 

 B, cerebral hemispheres ; 

 c, optic ganglia; D, cere- 

 bellum, so small as not to 

 cover the 4th ventricle, or 

 cavity left by the diver- 

 gence of the columns of 

 the Spinal Cord. 



