268 



FUNCTIONS OF THE NERVOUS SYSTEM. 



[Fig. 131. their posterior roots. Some of these 



last fibres also pass, with the ordi- 

 nary large nerve-tubes, from the 

 Cerebro-spinal into the Sympathetic 

 system. By these communications 

 the two systems of fibres are so 

 blended with each other, that it is 

 impossible to isolate them ; and all 

 that can be said is, that the large 

 tubular fibres predominate in the 

 former, and the fine homogeneous 

 fibres in the latter. 



342. The branches proceeding 

 from the Semilunar ganglia are dis- 

 tributed upon the abdominal viscera; 

 and those of the Cardiac ganglia 

 upon the heart and the vessels pro- 

 reeding from it. The latter seem 

 to accompany the arterial trunks 

 through their whole course, rami- 

 fying minutely upon their surface; 

 and it can scarcely be doubted, that 

 they exercise an important influ- 

 ence over their functions. What 

 the nature of that influence is, how- 

 ever, will be a subject for future 

 inquiry. It is so evidently con- 

 nected with the operations of nutri- 

 tion, secretion, &c., that the de- 

 signation of "nervous system of 

 organic life," as applied to this sys- 

 tem does not seem objectionable, 

 provided that we do not understand 



it as denoting the dependence of these functions upon it. Even in Vertebrata, 

 however, we do not always find the distribution of the visceral trunks distinct 

 from those of the cerebro-spinal. In the Cyclostome Fishes, the par vagum 

 supplies the intestinal canal along its whole length, as well as the heart; and 

 no appearance of a distinct sympathetic can be discovered. In Serpents, 

 again, the lower part of the alimentary canal is supplied from the spinal cord, 

 and the upper part by the par vagum ; and though the lateral cords of the 

 sympathetic may be traced, they are almost destitute of ganglia. Even in 

 the highest Vertebrata, some of the glands, of which the secretion is most 

 directly influenced by the condition of the mind, are supplied with most of 

 their nerves from the cerebro-spinal system ; thus, the lachrymal and sublin- 

 guul glands receive large branches from the fifth pair, and the mammary 

 glands from the intercostal nerves. But it appears probable, from what has 

 just been stated, that the influence is conveyed through the visceral fibres, 

 contained in these nerves, and either originating in the ganglia at their roots, 

 or derived from the Sympathetic system. 



343. The Spinal Cord, witli its encephalic continuation the Medulla Ob- 

 longata, may be regarded as constituting the essential part of the nervous 

 system of Vertebrata. Although the Cerebral Hemispheres in Man bear so 

 large a proportion to it in size, that the Spinal Cord seems but a mere ap- 

 pendage to them, the case is reversed when we look at the other extremity of 

 the scale; the Cerebral Hemispheres, in many Fishes, being but ganglionic 



Roots of a dorsal spinal nerve, and its union with 

 sympathetic : e, c. Anterior fissure of the spinal cord. 

 a. Anterior root. p. Posterior root, with its ganglion. 

 a'. Anterior branch, p' . Posterior branch, s. Sympa- 

 thetic, e. Its double junction with the anterior branch 

 of the spinal nerve by a white and a grey filament.] 



