514 



COMPARATIVE ANATOMY 



In the human brain the medulla is largely covered by the cerebellum. 

 To the cranial nerves connected with the medulla in the anamnia are 

 added the spinal accessory (XI) and the hypoglossal (XII). (Fig. 437) 



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 



AMPHIOXUS 



;a,.^ 



CeNfTRAL CANAL 



SOMATIC MOTOft 



Pig. 429. — Cross sections of the spinal cord of various vertebrates: — A, Amphioxus; 

 B, Petromyzon; C, Stjualus; £>, Rana; £, Alligator and F, Homo. The magnification 

 is not to the same scale. The section of the cord of Amphioxus is enlarged four times 

 as much as that of Petromyzon. In these two forms the axons are non-meduUated. 

 The striking differences between the cord of Amphioxus and that of man are bridged 

 over by intermediate conditions in lower vertebrates. 



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 am.phioxus 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 myelin nerve sheaths, amphi- 

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

 conclusion is substantiated by evidence from ontogenesis, which shows 



