434 GENEKAL ARKANGEMENT OP 



at the brain above, and terminating below in a conical point at the level 

 of the first lumbar vertebra. Its inner portions are occupied by gray 

 substance, which forms a continuous chain of ganglionic matter, run- 

 ning from one extremity of the cord to the other. Its outer portions 

 are composed of white substance, the fibres of which run mainly in a 

 longitudinal direction, connecting its different parts with each other, 

 and forming a communication between it and the brain. 



The spinal nerves are given off from the cord at regular intervals 

 and in symmetrical pairs ; one pair for each successive portion of the 

 ^ body, their branches being distributed to the integument and muscles 

 of the corresponding regions. In fish and serpents, where locomotion 

 is performed by means of simple, alternating, lateral movements of the 

 spinal column, the cord is nearly or quite uniform in size; or tapers 

 gradually from its anterior to its posterior extremity. But in the other 

 vertebrate classes, where the body is provided with special organs of 

 locomotion as fore and hind limbs, or wings and legs, the cord is in- 

 creased in size where the nerves of these organs are given off; and the 

 nerves supplying the limbs are larger than those which originate 

 from other parts of the cord. In man, the lower cervical nerves, 

 which form the brachial plexus and supply the arms, and the sacral 

 nerves forming the sacral plexus, which supplies the legs, are larger 

 than those given off in the upper cervical, dorsal, and lumbar regions. 

 The cord itself, furthermore, presents two marked enlargements cor- 

 responding with the points of origin of these nerves, namely, the cervical 

 enlargement, which is the source of the nerves for the upper extremity, 

 and the lumbar enlargement, which gives off the nerves destined for the 

 lower extremity. 



A transverse section of the spinal cord shows that it is incompletely 

 divided into right and left lateral halves by an anterior and posterior 



median fissure ; of which the 



Fig. 148. anterior is the wider and pene- 



trates for a comparatively short 

 distance, while the posterior is 

 narrower but extends inward 

 nearly or quite to the centre of 

 the cord. The gray substance 

 in the interior of the cord forms 

 a double crescentic-shaped mass, 

 w r ith the concavities of the 

 crescents turned outward. As 

 TRANSVERSE SECTION OP THE SPINAL these masses are found at all 



CORD, showing its central mass of gray sub- tg of the CQrd they haye in 



Btance, and the roots of the spinal nerves. a, b. 



Spinal nerves of right and left sides, d. Origin of reality the form of elongated 

 anterior root. e. Origin of posterior root. c. ribbons or bands o f gray sub _ 

 Ganglion of posterior root. 



stance, one on each side, run- 

 ning continuously throughout the length of the cord. The two are 

 united with each other by a transverse band of gray substance, known 



