ANATOMY OF THE SPINAL COED. 265 



tened in certain portions. It extends from the foramen 

 magnum to the first lumbar vertebra. It presents, at the 

 origin of the brachial nerves, an elongated enlargement, and 

 a corresponding enlargement at the origin of the nerves 

 which supply the lower extremities. It terminates below in 

 a slender, gray filament, called the filum terminale. The 

 sacral and coccygeal 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 formed of white and gray 

 matter, the white matter being external. The proportion 

 of white matter to the gray is greatest in the cervical region. 

 This fact is important in studying the course of the fibres 

 and in view of the functions of the cord as a conductor. 

 The inferior, pointed termination of the cord consists en- 

 tirely of gray matter. 



The cord is marked by .an anterior and a posterior me- 

 dian fissure, and by imperfect and somewhat indistinct an- 

 terior and posterior lateral grooves, from which arise the 

 anterior and the posterior roots of the spinal nerves. The 

 posterior lateral groove is tolerably well marked, but there 

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

 anterior median fissure, or sulcus, is perfectly distinct. It 

 penetrates the anterior portion of the cord in the median 

 line for about one-third of its thickness, and receives a high- 

 ly-vascular fold of the pia mater. It extends to the anterior 

 white commissure. The posterior fissure is not so distinct 

 as the anterior, and is not lined throughout by a fold of the 

 pia mater, but is filled with connective tissue and blood-ves- 

 sels, which form a septum posteriorly, between the lateral 

 halves of the cord. The posterior median fissure, so called, 

 extends nearly to the centre of the cord, to the posterior 

 gray commissure. 



Physiologically and anatomically, the cord is divided 

 into two lateral halves ; but the division of each half into 

 columns is not so distinct. Anatomists generally regard a 



