THE NERVOUS SYSTEM 



515 



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 sUtlike 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 ganghon cells is visible in the gray matter, ependymal cells being 

 the most abundant. Sensory ganglion cells connect with the dorsal 

 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 



DORSAL .NUCLEUS 



CCLARKES COLUMN)^ GROUND Bl*IDLD 



FASC. GRACILIS 



FASC CUNCATUS 



DORSAL ROOT 



SUBSTANTIA GELATINOSA 



POSTERIOR COLUMN 



LATERAL CELLS 



LAT COLUMN CELLS - 



MIDDLE CELLS -I 



•'OOHSO- LATERAL- 



INTERMEDIATE 



^yENTRO- LATERAL 



ANT \ VENTROMEDIAL 



COLUMN i~^ 



CELLS VENTRAL ROOT/! 



■sensory tract 

 lissauer's marginal zone 

 ~fasc. cerebello-spinalis 

 fasc. cerebro -spinalis lat. 



:. ANTEROLATERAL SUPEBFIC 

 -THALAMIC TRACT 

 HEUVEG'S BUNDLE 



AlsTTERIOR PR0PRIU3 

 ANTERIOR MARGINAL BUNDLE 



'^^^ DORSOMEDIAL' MARQINALia 



Fig. 430. — A diagram 01 a cross section of the spinal cord, showing the fiber tracts 

 or fascicuU, 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. 429) 



The spinal cord of cyclostomes is much flattened, with the cellular 

 matter distributed in a pair of lateral wings. Ventral to the central 

 lumen, a number of giant or Mueller's fibers extend lengthwise and carry 

 from the brain impulses thought to be chiefly static. Dorsal to the lumen, 

 are sensory ganghon cells Hke those of Amphioxus. The outer fibrous or 

 marginal layer of the cord is divided into longitudinal bundles of fibers, 

 the funiculi. Medullary sheaths are lacking, so that sympathetic fibers 

 can not be distinguished from others. Neurone relations in the cord 

 indicate that it is a reflex center and a pathway for intersegmental nervous 

 connexions. Increase in descending fiber tracts demonstrates the increas- 

 ing dominance of the brain. Polynuclear gland cells possibly of endocrinal 

 function, occur in the caudal region of the cord. (Fig. 429) 



