THE DISSECTION OF THE BACK 83 



cavity is the wide space between the arachnoid and pia mater. 

 It is occupied by a variable amount of cerebro-spinal fluid, and 

 is directly continuous with the cranial sub-arachnoid space 

 through the foramen magnum. Three incomplete septa 

 partially subdivide the spinal sub-arachnoid space into com- 

 partments. One of the septa is a median partition, called 

 the septum subarachnoideale, which connects the arachnoid with 

 the pia mater covering the posterior aspect of the spinal 

 medulla. In the upper part of the cervical region the sub- 

 arachnoid septum is represented merely by a number of 

 strands passing between the two membranes ; in the lower part 

 of the cervical region and in the thoracic region it is almost 

 complete. The other two septa are the ligamenta denticu- 

 lata. They spread laterally, one from each side of the medulla 

 spinalis, and will be studied with the pia mater. 



Dissection. Take away the arachnoid from a portion of the 

 spina medulla, and proceed to the study of the pia mater. 



Pia Mater Spinalis. The pia mater of the spinal medulla 

 is a firm vascular membrane, which adheres closely to the 

 surface of the medulla spinalis (O.T. spinal cord). It is 

 thicker and denser than the pia mater of the brain, largely 

 owing to the addition of an outer layer of fibres, which run 

 chiefly in a longitudinal direction. It is folded into the 

 antero - median fissure of the medulla spinalis; and the 

 posterior median septum of the medulla spinalis is firmly 

 attached to its deep surface. Anteriorly, in the median 

 plane, it is thickened to form a longitudinal glistening band, 

 which receives the name of the linea splendens. Of course, 

 that can be seen only after the medulla spinalis has been 

 removed from the vertebral canal. The blood vessels of the 

 medulla spinalis lie between the two layers of the pia mater 

 before they enter the substance of the spinal medulla ; and 

 the various spinal nerves receive from it closely fitting sheaths 

 which blend with their connective-tissue coverings. 



Ligamentum Denticulatum (Figs. 20 and 22). There are 

 two dentate ligaments, one on each side. Each stretches 

 laterally from the corresponding side of the medulla spinalis 

 and connects it with the dura mater. Its medial attachment 

 extends in a continuous line, between the anterior and 

 posterior nerve-roots, from the level of foramen magnum, 

 above, to the level of the body of the first lumbar vertebra, 



in 6 a 



