330 MEMBRANES OF THE SPINAL CORD. 



ligamenlum denticulatum on either side. The denticulate liga- 

 ment is a longitudinal band whose smooth medial border is con- 

 tinuous with the pia along the middle of the lateral surface of 

 the cord; its lateral border is notched and its twenty teeth, invested 

 with arachnoid, are attached to the dura opposite the first twenty 

 vertebrae. The two ligaments subdivide the space between the 

 pia and arachnoid into anterior and posterior subarachnoid spaces. 

 A filamentous extension of the pia below the cord proper helps 

 to form the filum terminate internum. It descends in the arachno- 

 dural sheath with the roots of the lumbar and sacral nerves, and 

 all together constitute the cauda equina (Fig. 100). For some 

 distance, about half its length, the filum terminale internum con- 

 tains gray matter and rudimentary fibers continuous with the 

 spinal cord. The filum unites with the arachnoid and dura at 

 the third sacral vertebra in forming the filum terminale externum 

 which forms a sort of ligament fbr^he spinal cord. That liga- 

 ment is inserted into the coccyx. The "pia mater of the cord con- 

 tains the trunks and large branches of the anterior and the two 

 posterior spinal arteries, and the tributaries of the external spinal 

 veins. 



Nerve Supply. The membranes of the spinal cord are sup- 

 plied by recurrent branches of the spinal nerves and by the sympa- 

 thetic. The recurrent branches are sensory in function. 



BLOOD SUPPLY OF THE SPINAL CORD. 



The vessels supplying the cord are the anterior spinal artery 

 and the two posterior spinal arteries, which rise at the foramen 

 magnum from the vertebral arteries, and are reinforced by cervical, 

 intercostal and lumbar arteries. The anterior spinal artery 

 (a. spinalis anterior) descends along the entrance to the anterior 

 median fissure (Fig. 99) ; it is formed by the union of two vessels, 

 one from each vertebral. The posterior spinal artery (a. spinalis 

 posterior) of either side, is in reality a pair of vessels which freely 

 communicate, and are so placed as to embrace the posterior nerve 

 roots. The larger vessel of the pair is anterior to the nerve roots, 

 while the smaller is between them and the posterior median fissure 



