NERVOUS SYSTEM OF VERTEBRATA. 



123 



1 B 



apex scarcely decussate ; but, in ascending towards 

 the higher classes, the communication between the 

 two sides is more intimate, and a larger proportion 

 of the pyramidal fibres crosses to the opposite sides. 

 In all the Vertebrata, the true Spinal Cord contains 

 gray substance, or something equivalent to it; thus 

 possessing the character of a continuous ganglion. 

 The proportion of the vertebral column which this 

 ganglionic portion occupies, is, however, extremely 

 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 extre- 

 mities, the true Spinal Cord scarcely passes out of 

 the cranium. The quantity of gray 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 prolongations of its vertebras), than 

 by the movements of its extremities, which serve prin- 

 cipally to guide it. Hence we usually find the amount 

 of gray matter 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 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 poste- 

 rior extremities, when these are developed ; at the same time becoming much 

 shortened, as the tail is less important to locomotion, or is altogether atrophied. 

 In Birds, the ganglionic en- 



Nervous Centres in Frog; 

 A, olfactive ganglia; B, cere- 

 bral hemispheres ; c, optic 

 ganglia; D, cerebellum, so 

 small as not to cover the 4th 

 ventricle, or cavity left by the 

 of the columns of 



largements are generally very 

 perceptible; and bear a close 

 relation in size, with the de- 

 velopment of the locomotive 

 organs with which they are 

 connected. Thus, in Birds of 

 active flight, and short power- 

 less legs,, the anterior enlarge- 

 ment 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 Struthi- 

 ous birds, the size of the pos- 

 terior enlargement is very re- 

 markable. Hence we have 

 a right to infer, that the in- 

 crease in the quantity of gray 

 matter in the cord has some 

 connection with the amount of 



Fig. 17. 



Transverse sections of human Spinal Cord at different 

 po ; nts. showing proportional quantity and arrangement of 

 gray and white mailer at each; after Solly: 1, opposite 11 ih 

 dorsal vertebra; 2, opposite 10th dorsal; 3, opposite Sth dor- 

 sal; 4, opposite 5th dorsal; 5, opposite 7th cervical; G, oppo- 

 site 4th cervical ; 7. opposite 3d cervical ; 8, section of medulla 

 pbloiigata through centre of corpus olivare. 



