THE SPINAL CORD 549 



border of the first lumbar vertebra. The sacral and cocygeal nerves, 

 after their origin from the lower portion of the cord, pass downward to 

 emerge by the sacral foramina and form what is known as the cauda 

 equina. The substance of the cord is composed of white and gray mat- 

 ter, the white matter being external. The pointed inferior extremity of 

 the cord consists entirely of gray matter. 



The cord presents an anterior and a posterior median fissure, 

 and imperfect and somewhat indistinct anterior and posterior lateral 

 grooves, from which latter arise the anterior and the posterior roots 

 of the spinal nerves. The posterior lateral groove is fairly well marked, 

 but there is no distinct line at the origin of the anterior roots. The 

 anterior median fissure is more distinct. It penetrates the anterior por- 

 tion of the cord in the median line for about one-third of its thickness 

 and receives a highly vascular fold of the pia mater. It extends to the 

 anterior white commissure. The posterior fissure is not so distinct as 

 the anterior. It is not lined throughout by a fold of the pia mater but 

 is filled with connective tissue and bloodvessels, which form a septum 

 posteriorly between the lateral halves of the cord. The posterior 

 median fissure extends nearly to the centre of the cord, as far as the 

 posterior gray commissure. 



The arrangement of the white and the gray matter in the cord is 

 seen in a transverse section. The gray substance is in the form of 

 a letter H, presenting two anterior and two posterior cornua connected 

 by what is called the gray commissure. The anterior cornua are short 

 and broad and do not extend to the surface of the cord. The posterior 

 cornua are longer and narrower, 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 with cells of 

 ciliated epithelium, called the central canal. This is in communication 

 above with the fourth ventricle and extends below to the filum termi- 

 nale. 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 central 

 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 sub- 

 stance is most abundant, and, in fact, it progressively increases in quan- 

 tity from below upward throughout 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, con- 



