CHAP. V 



SPINAL COED AND NEEYES 



279 



I. The spinal cord, which occupies 

 the whole extent of the vertebral canal 

 in the early months of foetal life, ex- 

 tends in the adult from the foramen 

 occipitale magnum to the lower edge 

 of the first lumbar vertebra, and has 

 an average length of 45 cm. (Fig. 167). 



There is a corresponding segment 

 or metamere of the spinal cord with 

 two pairs of nerves connected with it 

 for each segment of the vertebral 

 column. But the metamerism of the 

 roots must be distinguished from the 

 metamerism of the cord. The former 

 is a true and perfect metamerism, 

 because each pair of nerves (neuromere) 



FIG. 167. Diagrammatic view from before of spinal cord 

 and medulla oblongata, including the roots of the 

 spinal and some of the cranial nerves, and on one side 

 the gangliated chain of the sympathetic. (Allen 

 Thomson.) J. The spinal nerves are enumerated in 

 order on the right side of the figure. JBr, brachial 

 plexus ; O, anterior crural ; 0, obturator ; and Sc, great 

 sciatic nerves, coming off from lumbo-sacral plexus ; 

 X, x, filum terminate ; a, b, c, superior, middle, and 



I , inferior cervical ganglia of the sympathetic, the last 

 united with the 1st thoracic, d ; d', the llth thoracic 

 ganglion ; I, the 12th thoracic (or 1st lumbar) ; below 

 ss, the chain of sacral ganglia. 



is in relation at the periphery with 

 definite and circumscribed portions of 

 groups of muscles (myomeres) and 

 cutaneous areas (dermatomeres), as we 

 shall see in discussing the peripheral 

 distribution of the spinal nerves. In 

 the spinal cord, on the other hand, 

 metamerism is reduced to its lowest 

 terms. Originally independent, during 

 phylogenetic and ontogenetic evolution 

 the spinal segments (myelomeres) have 

 fused, and their functions have mingled. 

 What remains of their primary inde- 

 pendence is confined to the intimate 

 functional connection that exists in 

 carrying out the simplest reflex acts 

 between the ventral and the dorsal 

 roots of the same spinal segment. 



Fig. 168 shows the natural appear- 

 ance of a segment of the cord, with 

 the corresponding pair of spinal nerves 



a 12 



