124 FUNCTIONS OF THE NERVOUS SYSTEM. 



power to be supplied ; and this exactly corresponds with what has been 

 observed in the Articulated classes, and especially in watching tlje metamor- 

 phosis of Insects. In Birds and Mammalia, however, the whole amount of the 

 gray matter in the spinal cord does not bear so large a proportion to the bulk 

 of the nerves proceeding from it, as in the lower Vertebrata ; and the reason 

 of this seems obvious. The actions of the locomotive organs are less and less 

 of a reflex character, and are more directly excited by the will, and conse- 

 quently by the brain, than in the inferior tribes; and just in proportion, there- 

 fore, to the development of the Brain, will it become the centre of all the actions 

 performed by the animal, and the Spinal Cord a mere appendage to it. Still, 

 in all the Mammalia, even in Man, do we find these ganglionic enlargements 

 of the spinal cord ; and in Man it is the posterior (or rather the inferior) one 

 which contains the largest quantity of gray matter. In the cord of this class, 

 too, the lateral halves are much more intimately united than in the classes 

 below ; for not only is the central canal for the most part absent, but the two 

 crescent-shaped plates of gray matter are united by a transverse lamella, which 

 connects their centres like a commissure. 



165. The Cord is traversed, not only by the anterior and posterior fissures, 

 but by two furrows on each side, marking out three columns upon it. We 

 have, therefore, on each half of the cord, an anterior, middle or lateral, and 

 posterior column. The points of the crescentic lamellce of gray matter ap- 

 proach these furrows pretty closely ; but elsewhere the gray matter is covered 

 deeply by the fibrous columns. Each spinal nerve arises from two sets of 



roots. The anterior roots join the spinal 



Fig. IS. cord near the anterior furrow; and the 



posterior near the posterior furrow. 

 Respecting their intimate connection 

 with the principal divisions of the cord, 

 a considerable diversity has existed 

 among the statements of anatomists ; 

 but it seems to be now satisfactorily 

 ascertained, that, as in the Articulata, 



A transverse section of the Spinal Marrow.- a part Q f each rQot enters the mat . 



1,1. the two halves of the spinal marrow; 2 the / r i 



anterior middle fissure; 3, the posterior middle I- ^.P 1 S^gllOHW portion of the COrd, 



sure; 4, the position of the cineritious matter to each whl / st a P^rt IS COlltmUOUS With its 



half of the spinal marrow; 5, the origin of orfe of Whit* OY ftbrOUS Columns. The COUrSC 



the anterior roots of a spinal nerve ; 6, the origin of of tile fibres which enter the gray 



one of the posterior roots. matter, has been lately displayed, in 



part, at least, by the researches of Dr. 



Stilling.* It appears that of the fibres of the posterior roots, some form loops 

 in the gray matter, and become continuous with those of the anterior roots of 

 the same side, as seen at A, fig. 19. Others cross the gray matter, and become 

 continuous with those of the anterior roots of the opposite side, as seen at B. It 

 can scarcely be doubted that these fibres, being unconnected with the brain, 

 constitute the system to which reflex actions are due. Although Dr. Stillinp-'s 

 inquiries have not proved the fact,t yet it may be inferred from physiological 

 phenomena, as well as from the facts recently shown by Mr. Newport ( 143), 

 that there are other fibres, which pass from the posterior roots into the anterior 

 roots of other nerves above and below, both on the same and on the opposite 

 >f the portions of the roots which are continuous with the -fibrous 

 columns, it is stated by Sir C. Bell that the anterior fasciculi pass to the an- 



* Ueber die Textur und Function der Medulla Oblongata. 



- It may be thought that the mode of examination which he adopted, that of making 

 very thin transverse sections of the Spinal Cord,-is not well fitted to display the con- 

 nections of the roots with longitudinal fibres. 



