THE SPINAL CORD. 



663 



length, but presents an enlargement at the origins of the large nerves forming 

 the brachial and crural plexuses. This enlargement is 

 produced by an increase in the quantity both of the gray Fig. 175. 



and of the white substances; and it is obviously compar- ^ 



able to the enlargement of the ganglia of the ventral cord 

 of Articulata, which presents itself in connection with 

 the nerves of the special locomotive organs, as we well 

 see in tracing the alterations which this cord undergoes 

 between the "larva" and " imago" states of the Insect. 1 

 Its relation to the functions of these nerves is further 

 indicated by the constancy with which it presents itself, 

 through the entire Vertebrated series, in those parts of 

 the Cord from which the largest supply is transmitted to 

 the locomotive organs. In most Fishes, for example, the 

 body being propelled through the water rather by the 

 lateral action of the flattened trunk and tail (with that of 

 the median fins), than by the movements of the extremi- 

 ties, which serve principally to guide it, the size of the 

 Cord usually varies but little through its whole length ; 

 and this is especially the case with the Eel and 

 other vermiform apodal fishes. But in the Flying-fish, 

 and others whose pectoral fins are unusually powerful, a 

 distinct ganglionic enlargement of the cord presents itself 

 where their nerves are given off. In Serpents, again, 

 the spinal cord is nearly uniform throughout its entire 

 length; whilst in Amphibia it is so during the Tadpole 

 condition, but presents enlargements corresponding to the 

 anterior and posterior extremities, when these are deve- 

 loped; at the same time becoming much shortened, as the 

 tail is less important 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 anterior enlargement is 

 the principal; but in those which are more adapted to 

 run on land 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; 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 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 consequently by the brain, than in the inferior tribes; and just in 

 proportion, therefore, to the development of the Brain, will it become the mov- 

 ing spring of all the actions performed by the animal, and the Spinal Cord be 

 merely its instrument. Still, in all the Mammalia, as in Man, do we find these 



Transverse sections of 

 the spinal cord : A. Im- 

 mediately below the decus- 

 sation of the pyramids. 

 B. At middle of cervical 

 bulb. c. Midway between 

 cervical and lumbar bulbs. 

 D. Lumbar bulb. E. An 

 inch lower. F. Very near 

 the lower end. a. Anteri- 

 or surface, p. Posterior 

 surface. The points of 

 emergence of the anterior 

 and posterior roots of the 

 nerves are also seen. 



See "Princ. of Phys., Gen. and Comp.," g 776, Am. Ed. 



