602 THE NEEVOUS SYSTEM 



diameter (eight millimeters), and is very nearly circular in outline. 

 The white matter, since it contains the many nerve fibers going to and 

 coming from the lumbar enlargement, is much more voluminous than 

 the gray matter. The latter is reduced to a comparatively small central 

 mass. 



The dorsomedial column attains a considerable size in this region, 

 and is distinctly marked off from the adjacent dorsolateral column by 

 a fibrous septum derived in part from the pia mater, in part being 

 of neuroglia composition. The dorsal and the lateral columns, having 

 been much augmented by the influx of fibers from the large dorsal 

 roots of the lumbar nerves, form the larger part of the white matter. 

 The gray matter consequently appears to be pushed forward, its gray 

 commissure lies considerably ventrad of the center of the spinal cord, 

 the ventral median fissure is shorter than the dorsal median septum, and 

 the tips of the dorsal gray horns are far removed from the surface, 

 being only connected with the dorsolateral sulcus by the slender dorsal 

 nerve roots. In fact, the dorsal horns of gray matter in this region 

 are reduced to a minimum size; they are short and slender and contain 

 comparatively few nerve cells. 



At the base of each dorsal horn, on its mesial side, there is a dis- 

 tinctly outlined cell group whose transection is of oval or circular out- 

 line. Indeed, this cell group, the cell column of Clarke, begins in the 

 second or third lumbar segment, and is continued upward to the second 

 or third thoracic at times even into the lowermost cervical segments 

 at which level it has dwindled to a relatively insignificant group. In 

 the lower lumbar region an ill-defined group of cells occupying a similar 

 position and having the same function is known as the nucleus of 

 Stilling. 



The ventral gray horns are very short and narrow, and their cells 

 cannot be subdivided into groups as in the other regions of the spinal 

 cord. In the upper part of the thoracic region a distinct protuberance 

 makes its appearance at the base of the ventral horn, on its lateral 

 aspect. This is the precursor of the larger lateral horn of the cervical 

 region. In the upper thoracic region it contains a small cell group, the 

 dorsolateral. 



The nerve centers of the thoracic segments control the upper abdom- 

 inal region, the thorax, and the viscera. 



In the lower half of the cervical region the spinal cord presents 

 a distinct enlargement, within the gray matter of which are the nuclei 

 for the upper limbs. The spinal cord in this region is somewhat flat- 



