140 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



the dorsal to the brain (ascending and descending tracts), while 

 impulses travel in both directions in the lateral columns. 



In fishes the cord tapers regularly to the tip, but with the develop- 

 ment of legs in the terrestrial vertebrates the cord is considerably en- 

 larged where the nerves to the limbs are given off, the enlargements 

 bearing some ratio to the size of the limb. In the early stages the 

 spinal nerves leave the cord at right angles to its axis. With growth 



FIG. 145. Diagram of spinal cord and nerve roots; gray matter shaded. A, L, P; 

 anterior, lateral and posterior columns; Ap, anterior pyramidal tract; B, column of Burdach, 

 cc, central canal; ca, cl, cp, anterior, lateral and posterior cornua; Dr, dorsal root; Fa, Fp, 

 anterior and posterior fissures; G, column of Goll; g, ganglion of dorsal root; Ic, lateral 

 cerebellar tract; Ip, lateral pyramidal tract; sn, spinal nerve. 



the angle changes since the peripheral parts increase more in length 

 than does the cord. The result is that the posterior nerves are very 

 oblique and in the hinder part of the spinal canal they form a bundle 

 of parallel nerves, the cauda equina. Another result of the unequal 

 growth may be the drawing out of the hinder end of the cord into a 

 slender non-nervous thread, the filum terminale. 



The Brain. 



The spinal cord throws light on the extremely complex brain. 

 Here, as in the cord, there is primitively a tubular structure, with roof, 

 floor and sides, and with nerves connected with it which recall those 

 of the cord. In its development, as stated above, the brain from the first 

 is larger than the cord. It early has three enlargements separated by 

 two constrictions, the third enlargement passing gradually into the cord. 

 These enlargements are called, from in front backward, fore-brain, 



