258 



STUDIES IN PHYSIOLOGY 



section of the cord, apparently two kinds of material, known 

 as gray and white matter, can be distinguished. In the interior 

 of the cord is the gray matter in a form somewhat resembling 

 that of a capital H. The two projections of the H that 

 extend toward the ventral face of the spinal cord are called 

 the ventral horns of the gray matter ; the two dorsal horns 

 extend in the opposite direction to the outer surface of the 

 cord. Running across the cord and connecting the two 

 halves is a bridge of gray matter, the gray coin' mis-sure. 



The H -shaped 



A tfferefi '/ r/erres to 

 vcf 



faffer* 



Oorsa/ fissure 



Centra/ 

 f/ssure 



fiferert serves from 

 spincr/cortf 



mass of gray 

 matter is sur- 

 rounded by the 

 white matter 

 which consti- 

 tutes the re- 

 mainder of the 

 cord. 



Nerve Cells 

 and Fibers. 

 TJie unit of struc- 

 ture in the nerv- 

 ous system, as 

 in other tissues 



of the body, is the cell with its processes. Nerve cells, how- 

 ever, are more varied in form and more complex in structure 

 than any other cells in the body. When the ventral horns 

 of the gray matter are sufficiently magnified, one sees irregu- 

 lar bits of protoplasm, shaped more or less like triangles or 

 polygons. From the angles of each cell body project numer- 

 ous fine processes that look like tiny branching roots. These 

 are the protoplasmic processes. Another fiberlike process, how- 

 ever, has fewer branches than the others, and can be traced 

 for a considerable distance from the cell body. This is 

 called an axis cylinder process ; it is the beginning of a nerve 

 fiber. A short distance from the cell body the axis cylinder 



FIG. 117. Diagram of Cross Section of Spinal Cord. 



Showing the H-shaped gray matter, inclosed within 

 the white matter. Also a diagrammatic represen- 

 tation of the afferent, efferent, and connecting 

 fibers used in reflex action. 



