THE SPINAL COED 601 



which enter the dorsal and, later (through secondary neurons), the 

 lateral columns; thus both of these columns are of large size in and 

 above the lumbar region. The dorsomedial column in this region at- 

 tains an appreciable size, and a distinct pial septum marks its lateral 

 boundary. 



The spinal cord is now nearly circular in transection, its ventro- 

 dorsal being perhaps slightly less _^-* - ^- J ^~ 



than its transverse diameter (10 > "" : : ; N. 



mm.). The gray commissure lies -^ v $ 



very near the middle of the X^ \k 



spinal cord, and the ventral me- Jpg ' , ^ 



dian fissure is, therefore, quite as fi|$||j 

 deep as the dorsal median sep- 



t um - < ^'^^" - -' 



Both the ventral and dorsal '..- 



gray horns are long and thick. : 



Each dorsal horn contains a 

 large apical area of gelatinous ^^M^^ ^S^K^'-^ 



substance, is somewhat lonsrer on ^ 



FIG. 521. TRANSECTION OF THE SPINAL 



its lateral than on its medial CORD OF A CHILD, EIGHTH THORACIC 

 side, and reaches nearly to the SEGMENT. 

 dorsal surface of the spinal cord, Weigert stain. X 7. 



opposite the dorsolateral sulcus. 



The dorsal nerve roots entering at this level are apparently directed 

 toward the middle of the tips of the dorsal horns of gray matter; once 

 within the spinal cord they pass around to the mesial side of the dorsal 

 horns. 



The ventral horns, somewhat larger than the dorsal, present two 

 short and broad protuberances, the one at the ventromesial, and the 

 other and more prominent at the ventrolateral angle. A similar though 

 less prominent protuberance is seen at the base of the ventral horn, 

 on its lateral aspect. Each of these projections contains a more or 

 less well-defined group of motor nerve cells. The cell groups of the 

 ventral horns in the lumbar region are therefore a ventromedial, ventro- 

 lateral, and dorsolateral, together with an ill-defined central group oc- 

 cupying the deeper 'intermediate zone' of gray matter. 



The nerve centers contained in the lumbar region control the re- 

 flexes and musculature of the lower limbs and the lower part of the 

 abdominal wall. 



A transection of the spinal cord in the thoracic region is of small 



