672 



THE NEEVOUS SYSTEM 



Arachnoid 



Posterior nerve-root 



Spinal ganglion 



Anterior rainus 

 of nerve^ 



Posterior rainus 

 of nerve 



Dura mater 



Arachnoid 



Ligamentuiu denticulatum 



Dura mater 



Anterior nerve- 

 root (cut) 

 Posterior nerve- 

 root 



Anterior nerve- 

 root (cut) 



Ligarnentum 

 denticulatum 



Pia mater 



Anterior nerve-root 



through the foramen magnum with the posterior part of the wide subarachnoid 

 space of the spinal medulla. 



The cisterna pontis is the continuation upwards on the floor of the cranium of 

 the anterior part of the subarachnoid space of the spinal medulla. In the region 

 of the medulla oblongata it is continuous behind with the cisterna cerebello- 

 medullaris, so that this subdivision of the brain, like the spinal medulla, is 



surrounded by a wide sub- 

 arachnoid space. 



In front of the pons the 

 arachnoidea bridges across 

 between the projecting 

 temporal lobes, and covers 

 in the deep hollow in this 

 region of the brain. This 

 space is called the cisterna 

 interpeduncularis, and with- 

 in it are placed the large 

 arteries which take part in 

 the formation of the arterial 

 circle (of Willis). Leading 

 out from the interpedun- 

 cular cistern there are 

 certain wide subarachnoid 

 channels. Two of these are 

 prolonged into the lateral 

 fissures, and in these are 

 accommodated the middle 

 cerebral arteries. Anteriorlv 



FIG. 597. MEMBRANES OF THE SPINAL MEDULLA, AND THE MODE OF 



ORIGIN OF THE SPINAL NERVES. tne mterpeduncular cistern 



passes into a space in front 



of the optic chiasma (cisterna chiasmatis), and from this it is continued into the 

 longitudinal fissure above the corpus callosum. In this subarachnoid passage the 

 anterior cerebral arteries are lodged. 



The spinal part of the subarachnoid space is a very wide interval which is 

 partially subdivided into compartments by three incomplete septa. One of these 

 is a median partition called the septum posterius, which connects the pia mater 

 covering the posterior aspect of the spinal medulla with the arachnoid. In the upper 

 part of the cervical region the septum posterius is imperfect, and is represented 

 merely by some strands passing between the two membranes; in the inferior 

 part of the cervical region and in the thoracic region it becomes more com- 

 plete. The other two septa are formed by the ligamenta denticulata which spread 

 laterally one from each side of the spinal medulla. These will be described with 

 the pia mater. 



Granulationes Arachnoideales. When the surface of the dura mater is i 

 inspected after the removal of the calvaria, a number of small fleshy -looking ; 

 excrescences, purplish-red in colour, are seen ranged in clusters on each side of the 

 superior sagittal sinus, and when this sinus is opened they are also observed protrud- 

 ing in considerable numbers into its interior. These are the arachnoideal granu- 

 lations (O.T. Pacchionian bodies), and they are found also, in smaller numbers and 

 distinctly smaller size, in connexion with other blood-sinuses, such as the transverse 

 sinus, the straight sinus, and the cavernous sinus. At first sight they appear to 

 belong to the dura mater, but in reality they are projections from the arachnoid, i 

 In the child they are exceedingly small and rudimentary, and it is only as life 

 advances that they become large and conspicuous. 



Each granulation is a bulbous protrusion of the arachnoid. It is attached to 

 the arachnoid by a narrow pedicle, and into its interior there is prolonged, through ;j; 

 this pedicle, a continuation of the subarachnoid space and its characteristic mesh- 

 work. The granulations do not pierce the dura mater. As they push their way jf 

 into a blood-sinus they carry before them a thin covering continuous with the 



