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THE NERVOUS SYSTEM 



In the early foetus the spinal nerves pass from their attachment to the spinal 

 cord outward through the intervertebral foramina at right angles to the long axis 

 of the cord, but, owing to the fact that the vertebral column increases consider- 

 ably in length after the spinal cord has practically ceased growing, the nerve-roots 

 become drawn caudad from their points of attachment, and, as is necessarily 

 the case, their respective foramina are displaced progressively downward as the 

 termination of the cord is approached, until finally the roots of the lumbar and 

 sacral nerves extend downward as a brush of parallel bundles considerably 

 below the levels at which the}- are attached. This brush of nerve-roots is the 

 Cauda equina. The dura mater, being more closely related to the bony wall of the 

 canal than to the spinal cord, extends with the vertebral column and thus en- 

 velops the Cauda equina, undergoing a slightly bulbous, conical dilation which 

 decreases rapidly and terminates in the attenuated canal of the coccyx as the 

 coccygeal ligament. 



The enlargements. — Wherever there is a greater mass of tissue to be in- 

 nervated, the region of the nervous system supplying such must of necessity 

 possess a greater number of neurones. Therefore, the regions of the spinal cord 

 associated with the skin and musculature of the regions of the superior and 



Fig. 612. — Dorsal View of Portion of Spinal Cord in Position in Vertebral Canal 



Dura mater spinalis -- 



Araclinoidea spinalis 

 -■' Lower cervical region 



*^ Spinal nerve 



Thoracic region 



) 



inferior limbs are thicker than the regions from which the neck or trunk alone are 

 innervated. Thus in the lower cervical region the spinal cord becomes broadened 

 into the cervical enlargement, and likewise in the lumbar region occurs the lumbar 

 enlargement. The spinal nerves attached to these regions are of greater size 

 than in other regions. 



The cervical enlargement [intumescentia cervicalis] begins with the third 

 cervical vertebra, acquires its greatest breadth (12 to 14 mm.) opposite the lower 

 part of the fifth cervical vertebra (origin of the sixth cervical nerves), and extends 

 to opposite the second thoracic vertebra. Unlike the lumbar enlargement, its 

 lateral is noticeably greater than its dorso-vcntral diameter. 



The lumbar enlargement [intumescentia lum])alis] begins gradually with the 

 ninth or tenth thoracic vertebra, is most marked at the twelfth thoracic vertebra 

 (origin of the fourth lumbar nerves), and rapidly diminishes into the conus 

 meduUaris. 



Both the lumbar and thoracic regions are practically circular in transverse section. Neither 

 diameter of the lumbar is ever so great as the lateral diameter of the cervical enlargement. 

 The thoracic part attains its smallest diameter opposite the fifth and sixth thoracic vertebra 

 (attachment of the seventh and eighth thoracic nerves.) 



The enlargements occur with the development of the upper and lower limbs. In the embyro 

 they are not evident until the limbs are formed. In the orang-utan and gorilla the cervical 

 enlargement is greatly developed; the ostri(-h and emu have practically none at all. 



Surface of the spinal cord.-— The cord is separated into nearl}^ symmetrical 

 right and left halves by t lie hroad anterior median fissure into which the pia mater 

 is duplicated, and opposite this, on the dorsal surface, by the posterior median 

 sulcus. Along the lower two-thirds of the cord this sulcus is shallowed to little 



