PHYSIOLOGICAL ANATOMY OF THE SPINAL CORD. 591 



reach to the surface of the cord. The posterior cornua are larger and nar- 

 rower, and they extend nearly to the surface, at the point of origin of the 

 posterior roots of the spinal nerves. In the centre of the gray commissure, is 

 a narrow canal, lined by cells of ciliated epithelium, called the central canal. 

 This is in communication above with the fourth ventricle, and it extends be- 

 low to the filum terminale. That portion of the gray commissure situated in 

 front of this canal is sometimes called the anterior gray commissure, the 

 posterior portion being known as the posterior gray commissure. The cen- 

 tral canal is immediately surrounded by connective tissue. In front of the 

 gray commissure, is the anterior white commissure. 



The proportion of the white to the gray substance is variable in different 

 portions of the cord. In the cervical region, the white substance is most 

 abundant, and in fact it progressively increases in quantity from below up- 

 ward throughout the whole extent of the cord. In the dorsal region, the 

 gray matter is least abundant, and it exists in greatest quantity in the lumbar 

 enlargement. 



The white substance of the cord is composed of nerve-fibres, connective- 

 tissue elements (neuroglia) and blood-vessels, the latter arranged in a very 

 wide and delicate plexus. The nerve-fibres are variable in size and are com- 

 posed of the axis-cylinder and the medullary substance, without the tubular 

 membrane. 



The anterior cornua of gray matter contain blood-vessels, connective-tis- 

 sue elements (neuroglia), very fine nerve-fibres, and large multipolar nerve- 

 cells, which are sometimes called motor cells. The posterior cornua are com- 

 posed of the same elements, the cells being much smaller, and the fibres ex- 

 ceedingly small, presenting very fine plexuses. The cells in this situation 

 are sometimes called sensory cells. Near the posterior portion of each poste- 

 rior cornu, is an enlargement, of a gelatiniform appearance, containing 

 small cells and fibres, called the substantia gelatinosa,' The connections 

 between the nerve-eells and the nerve-fibres have already been described in 

 connection with the general structure of the nervous system. The multi- 

 polar nerve-cells are supposed to present certain prolongations which do not 

 branch and are directly connected with the medullated nerve-fibres. These 

 are called axis-cylinder prolongations. In addition, fine, branching poles are 

 described under the name of protoplasmic prolongations. In both the white 

 and the gray substance of the cord, is a ground-work of delicate connective- 

 tissue fibres and cells, called neuroglia. This supports the nerve-cells, nerve- 

 fibres, vessels etc. The neuroglia is particularly abundant in that part of the 

 posterior cornua of gray matter, called the substantia gelatinosa. 



The division of the spinal cord into columns has a physiological as well 

 as an anatomical basis. Anatomists usually recognize, on either side of the 

 cord, an anterior column, bounded by the anterior median fissure and the 

 line of origin of the anterior roots of the spinal nerves, a lateral column, 

 bounded by the lines of origin of the anterior and of the posterior roots of 

 the nerves, and a posterior column, bounded by the line of the posterior roots 

 of the spinal nerves and the posterior median fissure. As the anterior or 



