THE BRAIN 



147 



spinal cord becomes enlarged in the regions where the nerves for the limbs are 

 given off; this reaching possibly the extreme in certain fossil reptiles where casts 

 of the spinal canal indicate that there was an accumulation of nervous matter 

 near the hind legs which exceeded the brain in size. 



In the early stages the nerves leave the spinal cord at nearly right angles to 

 its axis. Then there occurs an inequality in growth, the body increasing more 

 in length than does the cord. As a result the more posterior nerves pursue a 

 very oblique course and in the hinder part of the spinal canal of the higher verte- 

 brates they form a bundle of parallel nerves, the cauda equina (horse-tail). 

 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 tenninale. 



The Brain 



A knowledge of the structure of the spinal cord throws light upon 

 the greatly more complex brain, for the latter is primitively a tubular 



Fig. 155. — Diagrams of median sections of (i) primitive brain; (2) an intermediate 

 •tage, and (3) with the definitive parts. (Compare 3 with fig. 156). AQ, aqueduct, 

 AC, anterior commissure; C, cerebral region; CB, cerebellum; C5, corpus striatum; HC; 

 habenular commissure; /, infundibulum; Z,r, lamina terminalis; MO, medulla oblongata, 

 O, olfactory region; P, epiphysial region; PC, posterior commissure;»|/?0,loptic recess; 

 ST, subthalamica; T, tegmentum; TH, thalamus. Dorsal zone plain, ventral zone 

 dotted; F, M, H, fore-, mid- and hind-brains. 



Structure, with roof, floor and^sides,'which at first closely resemble 

 those of the cord. It has also nerves connected with it which recall 

 those of the spinal cord, to be described later. 



