i86 HEAD AND NECK 



is directly continuous with the cranial sub-arachnoid space. 

 Three incomplete septa partially subdivide the spinal sub- 

 arachnoid space into compartments. One of these septa is a 

 median partition called the septum subarachnoideale^ which con- 

 nects the pia mater covering the posterior aspect of the spinal 

 medulla with the arachnoid. In the upper part of the cervical 

 region the subarachnoid 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 two other septa are formed by the liga- 

 menta denticulata. These spread outwards from each side of 

 the medulla spinalis, and will be studied with the pia mater. 



Dissection. Take away the arachnoid mater from a portion of the spinal 

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



Pia Mater Spinalis. This is a firm vascular membrane, 

 closely adherent 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. The 

 pia mater sends a fold into the antero-median fissure of the 

 medulla spinalis, and the septum which occupies the posterior 

 median fissure of the medulla spinalis is firmly attached to 

 its deep surface. Anteriorly, in the median plane, the pia 

 mater is thickened to form a longitudinal glistening band, 

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

 this can be seen only after the medulla spinalis (O.T. spinal 

 cord) 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 sheaths. 



Ligamentum Denticulatum (Figs. 74 and 75). This is a 

 remarkable band, which stretches laterally from either side 

 of the medulla spinalis, and connects it with the dura mater. 

 Its pial or 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 below. Its lateral margin is widely serrated or 

 denticulated. From twenty to twenty-two denticulations may 

 be recognised, and the highest is attached to the margin of 

 the foramen magnum. They occur in the intervals between the 



