NERVOUS TISSUES AND SYSTEMS. 83 



The surface of the spinal cord comprises four areas : Anterior, 

 posterior, and two lateral areas. The cord is divided vertically into 

 two lateral halves by the anterior median fissure and the posterior 

 median septum. These fissures do not extend through the com- 

 missures, 



Each half of the spinal cord is divided into the anterior column, 

 posterior column, and lateral column. These divisions are marked 

 by furrows indicating the exit of the anterior and posterior roots of 

 the spinal nerves. In the upper thoracic and lower cervical regions, 

 two divisions appear in the posterior column, a median portion, 

 called the column of Goll, and a lateral portion, the column of Bur- 

 dock. 



The white matter is held together by septa of connective tissue 

 which proceed from the pia mater. Under the microscope it appears 

 to be made up of a vast number of nerve fibres, exhibiting an axis 

 cylinder, as a central dot, surrounded by a lighter substance, the 

 medullary sheath. 



The gray substance of the spinal cord is centrally located, and 

 appears in the form of the letter H i. e., two irregular bands con- 

 nected by a bridge. The bridge consists of the anterior gray com* 

 missure and the posterior gray commissure. Between the two com- 

 missures is the central canal. This extends through the cord longi- 

 tudinally, and is lined with ciliated epithelium. It is about 1mm 

 in diameter. The commissures consist of fibres derived from the 

 commissure cells. The white commissure is immediately in front of 

 the anterior gray commissure. Each lateral column of the gray 

 substance consists of three well-marked divisions the anterior, 

 posterior, and lateral cornua. From the anterior cornua emerge the 

 anterior roots, while the posterior roots enter the posterior cornua. 

 The reticular process, column of Clark, and the substantia gela-* 

 tinosa enter into the structure of a posterior cornu. The reticular 

 process is a net-like mass of gray substance at the base of the cornu. 

 The column of Clark is on the median side, near the gray com- 

 missure. The substantia gelatinosa covers the horn and immedi- 

 ately surrounds the central canal. The substantia gelatinosa Neuro- 

 landi; zona spongiosa, and zona terminalis are also found in the 

 posterior cornu. 



