SPIJCAL CORD OF VERTEBRATA. 



271 



ties, the true Spinal Cord scarcely passes out of the cranium. The quan- 

 tity of grey matter is nearly uniform in every part of the cord, where 

 there is no great diversity in the functions of the nerves which originate 

 from each portion. In most Fishes, for example, the body is propelled 

 through the water more by the lateral action of the flattened trunk (whose 

 surface is extended by the dorsal and caudal fins erected upon prolonga- 

 tions of its vertebrae), than by the movements of its extremities, which serve 

 principally to guide it. Hence we usually find the amount of grey mat- 

 ter varying but little in different parts of the cord. But in the Flying-fish, 

 and others whose pectoral fins are unusually powerful, a distinct ganglionic 

 enlargement of the cord takes place where the nerves are given olf. In Ser- 

 pents, again, the spinal cord is nearly uniform throughout its entire length; 

 whilst in Amphibia it is so during the Tadpole condition, but presents enlarge- 

 ments corresponding to the anterior and posterior extremities, when these are 

 developed; at the same time becoming much shortened, as the tail is less im- 

 portant to locomotion, or is altogether atrophied. In Birds, the ganglionic 

 enlargements are generally very perceptible ; and bear a close relation in 

 size, with the development of the locomotive organs with which they are 

 connected. Thus, in birds of active flight, and short powerless legs, the an- 

 terior enlargement is the principal ; but in those which are more adapted to 

 run on hand than to wing their way through the air, such as the whole tribe 

 of Struthious birds, the size of the posterior enlargement is very remarkable. 

 Hence we have a right to infer, that the increase in the quantity of grey mat- 

 ter in the cord has some connection with the amount of power to be supplied ; 

 and this exactly corresponds with what has been observed in the Articulated 

 classes, and especially in watching the metamorphosis of Insects. In Birds 

 and Mammalia, however, the whole amount of the grey 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 consequently by the brain 

 than in the inferior tribes ; and 

 just in proportion, therefore, 

 to the development of the 

 Brain, will it become the cen- 

 tre 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 gan- 

 glionic enlargements of the 

 spinal cord; and in Man it is 

 the posterior one (or rather the 

 inferior), which contains the 

 largest quantity of grey mat- 

 ter. 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 grey matter are united by a transverse 

 lamella, which connects their centres like a commissure. 



347. The Cord is transversed, not only by the anterior and posterior fis- 



Fisr. 133. 



Transverse sections of human Spinal Cord at different 

 points, showing the proportional quantity and arrangement 

 of grey and white matter at each: 1. opposite llth dorsal ver- 

 tebra; 2, opposite 10th dorsal ; 3, opposite 8th dorsal; 4, op- 

 posite 5th dorsal ; 5, opposite 7th cervical ; 6, opposite 4th 

 cervical; 7, opposite 3d cervical; 8, section of medulla ob- 

 longata through centre of corpus olivare. 



