106 HUMAN PHYSIOLOGY. 



SPINAL CORD. 



The Spinal Cord varies from 16 to 18 inches in length ; is half an inch 

 in thickness, weighs l^ oz., and extends from the atlas to the 2d lumbar 

 vertebra, terminating in the filum terminate. It is cylindrical in shape, 

 and presents an enlargement in the lower cervical and lower dorsal regions, 

 corresponding to the origin of the nerves which are distributed to the 

 upper and lower extremities. The cord is divided into two lateral halves 

 by the anterior and posterior fissures. It is composed of both white or 

 fibrous and gray or vesicular matter, the former occupying the exterior of 

 the cord, the latter the interior, where it is arranged in the form of two 

 crescents, one in each lateral half, united together by the central mass, the 

 gray commissure ; the white matter being united in front by the white 

 commissure. 



Structure of the White Matter. The white matter surrounding each 

 lateral half of the cord is made up of nerve fibres, some of which are con- 

 tinuations of the nerves which enter the cord, while others are derived 

 from different sources. It is subdivided into: (l) An Anterior column, 

 comprising that portion between the anterior roots and the anterior fissure, 

 which is again subdivided into two parts : (a) an inner portion, bordering 

 the anterior median fissure, the direct pyramidal tract, or column of Tiirck, 

 containing motor fibres which do not decussate, and which extends as far 

 down as the middle of the dorsal region ; (b] an outer portion, surrounding 

 the anterior cornua, known as the anterior root zone, composed of short 

 longitudinal fibres which serve to connect together different segments of 

 the spinal cord. (2) A Lateral column, the portion between the anterior 

 and posterior roots, which is divisible into [a] the crossed pyramidal tract, 

 occupying the posterior portion of the lateral column, and containing all 

 those fibres of the motor tract which have decussated at the medulla ob- 

 longata; it is composed of longitudinally running fibres which are con- 

 nected with the multipolar nerve cells of the anterior cornua ; (<$) the 

 direct cerebellar tract, situated upon the surface of the lateral column, 

 consisting of longitudinal fibres which terminate in the cerebellum ; it first 

 appears in the lumbar region, and increases as it passes upward; (c) the 

 anterior tract, lying just posterior to the anterior cornua. (3) A Posterior 

 column, the portion included between the posterior roots and the posterior 

 fissure, also divisible into two portions, (a) an inner portion, the postero- 

 internal column, or the column of Goll, bordering the posterior median 

 fissure, and (b} an external portion, the postero-external column, the 

 column of Burdach, lying just behind the posterior roots. They are com- 



