SPINAL CORD 175 



temporal bone and the foramen magnum respectively. On the 

 lower surface of the medulla they anastomose together to 

 form the circle of Willis, from which branches are derived 

 which supply freely the different parts of the brain. 



There are no veins proper in the brain ; the venous blood is 

 collected in the blood-spaces or sinuses in the dura mater, which- 

 finally leave the cranium by the jugular foramina as the internal 

 jugular veins. 



The Spinal Cord. 



The spinal cord occupies the upper two-thirds of the verte- 

 bral canal. It extends from the margin of the foramen magnum 

 of the occipital bone to the level of the upper border of the 

 second lumbar vertebra. At its upper end it is continuous with 

 the medulla oblongata of the brain, at its lower end it tapers, 

 forming a pointed extremity called the conus medullaris, from 

 the end of which comes a slender thread called the filum ter- 

 minale. The cord is a cylindrical structure, slightly flattened 

 before and behind, and is considerably smaller than the canal, 

 which allows movements of the vertebral column to take place 

 without jarring the cord. 



The cord has three coverings continuous with those of tne 

 farain viz., dura mater, arachnoid mater, and pia mater. The 

 cord is suspended within the dura mater by two ligaments, which 

 project laterally in its whole length, called the ligamenta dentic- 

 ulata. These extend outwards, and are attached to the inner 

 surface of the dura mater by tooth-like projections. 



The cord is not of uniform thickness throughout, but is con- 

 siderably thickened in the lower cervical and upper lumbar 

 regions. The cord similarly to the brain has a longitudinal fissure 

 running down its posterior surface, which grooves it nearly to 

 the centre. There is a shallower anterior furrow, and two very 

 shallow grooves, one on each side of the posterior crus. These 

 are called the postero-lateral grooves. 



On transverse section the cord is seen to consist also of grey 

 and white matter, but the grey matter is entirely embedded in 

 the white, and has a minute central canal running through 

 it, which communicates above with one of the ventricles of 

 the brain. The grey matter forms a column which extends 



