SPINAL CORD. 383 



Each spinal nerve has two roots — a dorsal, posterior, or 

 sensory, and a ventral, anterior, or motor. These arise sepa- 

 rately and independently, but combine in the vicinity of the 

 cord to form a single nerve. The posterior root exhibits at 

 an early period a large ganglionic swelling — the spinal 

 ganglion ; the anterior root is apparently nonganglionated, 

 but some modification of this statement will be found below. 

 Moreover, the posterior or dorsal root has always a 

 single origin (as in the cranial nerves), while that of the 

 anterior or ventral root is often multiple. 



But consider the origin of the spinal nerves more precisely. The 

 posterior roots are outgrowths of a continuous ridge or crest along the 

 median dorsal line of the cord. As the cord grows the nerve-roots of 

 each side become separated. They shift sidewards and downwards, 

 and acquire a secondary attachment to the side of the cord, while the 

 primary attachment disappears. The anterior roots are later in arising ; 

 they spring from the latero-ventral angle of the cord ; they retain their 

 original attachment. 



Beard has shown in several cases that the spinal ganglia have an in- 

 dependent epiblastic origin by the sides of the medullary canal. 



According to some authorities, the sympathetic ganglia are off-shoots 

 from the same rudiment as that from which the posterior or dorsal ganglia 

 arise, and it is possible that they are the more or less vagrant ganglia of 

 the anterior or ventral roots, with which they are connected by small 

 fibres. On this view (Gaskell's) both roots may be said to be ganglion- 

 ated. But the ganglion of the dorsal or posterior root is stationary in 

 position, and the nerve fibres which pass through it come both from the 

 visceral (splanchnic) and from the peripheral parts (somatic), separating 

 from one another within the cord. On the other hand, the supposed 

 ganglion (sympathetic) of the ventral or anterior root is more or less 

 vagrant, and off the main line of the root, from which it receives small 

 fibres passing to splanchnic or visceral structures. 



In the spinal cord it is usually easy to distinguish ai^ 

 external region of white matter or medullated nerve- 

 fibres from an internal region of gray matter or ganglionic 

 cells. The minute central canal is lined by ciliated epi- 

 thelium. 



The central canal is continuous with the cavity of the 

 brain, and is a remnant of the original medullary canal. 

 It can hardly be said to have any function ; it may be 

 simply the result of a developmental necessity. But 

 Sutton and Gaskell have independently suggested that the 

 central canal of the nervous system represents a disused 



