THE SPINAL CORD 827 



nerve emerges. Very inconsistently the pair emerging between the seventh cervi- 

 cal and first thoracic vertebrae is called the/eighth cervical pair. The remaining 

 spinal nerves are named after the upper of" the two vertebrae forming the corre- 

 sponding foramen. In all there are: 



Cervical pairs 8 



Thoracic pairs 12 



Lumbar pairs 5 



Sacral pairs 5 



Coccygeal pairs 1 



31 



All spinal nerves are made up of two roots by which they spring from the lateral 

 aspects of the cord, symmetrically arranged, and these nerve root attachments 

 are the only guides to the demarcation of the various segments of the spinal cord. 

 The two roots are generally termed the posterior or dorsal (afferent or sensor) 

 root, which enters the cord along the dorsolateral fissure, and the anterior or ventral 

 (efferent or motor) root, which emerges along the ventrolateral fissure. 



Although the cervical nerves pass outward through the intervertebral foramina 

 at nearly a right angle to the long axis of the cord, those of the lower series slope 

 more and more caudad or do\vnward, so that the fifth lumbar pair emerges six 

 vertebral bodies lower than it originates. In fact, the lumbar and sacral nerves 

 descend as parallel bundles in a brush-like manner to form the cauda equina, 

 enclosed by the dural sheath as far as about the middle of the sacral canal. The 

 topographical relations of the levels of origin and exit of the spinal nerves to the 

 spinous processes of the vertebra are shown in Fig. 597. 



Corresponding with the degree of development of the periphery, the spinal cord 

 is more massive in those segments which are associated with the limbs. Thus, 

 in the ground-mole, the cervical portion is very much enlarged in conformity 

 with the powerfully developed forelimbs, while in the kangaroo or the ostrich, 

 with powerful legs, the lumbar portion of the spinal cord is proportionately 

 enlarged. In man, both the cervical and lumbar portions are enlarged, and while 

 the bulk of the lower limbs exceeds that of the upper, the cervical enlargement 

 of the spinal cord is greater because it innervates a limb which is functionally 

 more differentiated, capable of much more skilful and complex movements, and 

 endowed with more acutely developed tactile sensibility. 



The Enlargements of the Spinal Cord. The spinal cord is marked by two 

 spindle-shaped enlargements in its cervical and lumbar portions, while the inter- 

 vening thoracic portion is nearly cylindrical, being slightly reduced in its antero- 

 posterior diameter. The cervical enlargement (intumescentia cervicalis) extends 

 from the first or second cervical segment to the level of the second thoracic 

 vertebra, acquiring a maximum breadth (13 to 14 mm.) at the sixth cervical ver- 

 tebra. At its junction with the medulla oblongata its breadth is about 11 mm. 

 The thoracic portion is about 10 mm. in breadth (minimum at a little below its 

 middle) while its sagittal diameter is 8 mm. The lumbar enlargement (intumescen- 

 tia lumbalis) begins at the level of the tenth thoracic vertebra, and acquires its 

 maximum breadth (12 mm.; sagittal diameter = 9 mm.) opposite the twelfth 

 thoracic vertebra. Below the lumbar enlargement the cord gradually tapers to 

 form a cone (conus), the apex of which, at the level of the lower border of the 

 body of the first lumbar vertebra, is continuous with the attenuated filum. 



The cervical enlargement is characterized by a relatively greater breadth than 

 the remaining portions of the cord which, on section, appear nearly circular. 



Conus (conus medullaris). The conus is the conical extremity of the cord. 

 The lower three sacral segments and the coccygeal segment are usually included 



