THE NERVOUS SYSTEM 367 



the brain, the posterior commissure, marks the boundary between (Hen- 

 cephalon and mesencephalon. A dorsal commissure in the roof of the 

 mid-brain connects the optic lobes. 



In addition to the commissures, the fibers of two cranial nerves, the 

 optic and the trochlearis, cross the median plane to form chiasmas. 

 The optic chiasma is ventral and just anterior to the infundibulum. The 

 trochlearis chiasma, which occurs in all vertebrates except cyclostomes, 

 lies in the dorsal constriction which separates mes- and metencephalon. 



The dorsal commissure is lacking in fishes. In amphibians and reptiles 

 a dorsal pallial or hippocampal commissure, connecting right and left 

 hippocampi, adds a third to those located in the terminal lamina of the 

 brain. In tnonotremes and marsupials both anterior and posterior pallial 

 or hippocampal commissures occur. The corpus callosum of placental 

 mammals is a new commissure — possibly derived from the anterior pallial 

 — which connects the two halves of the neocortex. Its enlargement is 

 correlated with that of the cerebral cortex. 



EVOLUTION OF THE SPINAL CORD 



The spinal cord is a much more conservative portion of the central 

 nervous system than the brain. Consequently, although the contrast 

 between the so-called brain of Amphioxus and that of man is so very great 

 that their homology may be doubted, the spinal cords of these chordate 

 extremes are recognizably similar. Both are tubular and both have a 

 central mass of gray matter and an external layer of fibrous tissue. The 

 relations of dorsal and ventral nerve roots are similar. The differences 

 are bridged over by intermediate conditions in the vertebrate series. 



The fact that the brain of Amphioxus differs from its spinal cord, not in 

 greater size but chiefly in the expansion of its lumen, appears to support 

 the inference that spinal cord and brain were originally undifferentiated 

 from one another. The same columns of gray matter are recognizable 

 in both, although, because of the absence of myeUn nerve sheaths, Amphi- 

 oxus lacks the color contrast between white and gray matter. This 

 conclusion is substantiated by evidence from ontogenesis, which shows 

 that the distribution of nervous matter is essentially similar in cord and 

 brain. 



The spinal cord of Amphioxus is somewhat triangular in cross section, 

 with the apex of the triangle dorsal, the base resting upon the notochord. 

 The small amount of gray matter lies close to the sHt-like central canal. 

 The lateral walls are much thicker than the dorsal and ventral, since 

 fibrillar material which makes up most of the substance of the cord is 

 wholly lateral in position. Some cellular differentiation into sensory and 

 motor ganglion cells is visible in the gray matter, ependymal cells being 

 the most abundant. Sensory ganglion cells connect with the dorsal 



