368 



CHORDATE ANATOMY 



nerves, and giant neurochord cells extend across the central canal. The 

 fibers of the giant nerve cells, after decussation, extend lengthwise of the 

 cord, some caudad and some cephalad. Since these do not form nerve 



'CENTTRAL CA^aAL 



Fig. 326. — Cross sections of the spinal cord of various vertebrates: — A, Amphioxus; 

 B, Petromyzon; C, Squalus; D, Rana; E, 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. 



DORSAL NUCLEUS ,FASC GRACiLIS 



(CLARKE^ COLUMN}^ GROUND BUNOLD 



DORSAL ROOT 

 SUBSTANTIA CELATINOSA^ 

 POSTERIOR COLUMN 

 LATERAL CELLS 

 LAT COLUMN CELLS - 

 MIDDLE CELLS - 

 !'DORSO-LATERAL- 



FASC. CUNEATUS 



INTERMEDIATE 



^yENTRO- LATERAL 



ANT. \ VENTROMEDIAL 



COLUMN '-^ 



CELLS 



SENSORY TRACT 

 lissauer's marginal ZONE 



"^ASC CEREBELLO -SPINALIS 

 - FASC CEREBRO-SPINALIS LAT 



ventral root/-- 



^^ dorsomedial' 



fasc anterolateral superfic 

 spino-thalamic tract 

 "helweg's bundle 

 fasc anterior pr0priu3 

 anterior marginal bundle 



Fig. 327. — A diagram of a cross section of the spinal cord, showing the fiber tracts 

 or fasciculi, and the arrangement of nuclei in the gray matter. (Redrawn after 

 Sobotta.) 



fibers which leave the cord, it is supposed that the giant fibers give off 

 collaterals to the motor centers along the cord, and serve to correlate their 

 activities. (Fig. 326) 



