574 



CIRCULATION OF BLOOD AND LYMPH. 



The Meningeal Spaces. The general arrangement of the menin- 

 geal membranes, and particularly of the meningeal spaces, is im- 

 portant in connection with the mechanics of the brain circulation. 

 In the skull the dura mater adheres to tke bone, the pia mater 

 invests closely the surface of the brain, while between lies the 

 arachnoid (Fig. 236). The capillary space between the arachnoid 

 and the dura, the so-called subdural space, may be neglected. 

 Between the arachnoid and the pia mater, however, lies the sub- 

 arachnoidal space more or 

 less intersected by septa of 

 connective tissue, but in free 

 communication throughout 

 the brain and cord. This 

 subarachnoidal space is filled 

 with a liquid, the cerebro- 

 spinal liquid, which forms a 

 pad inclosing the brain and 

 cord on all sides. The liquid 

 surrounding the cord is in 

 free communication with 

 that in the brain, as is indi- 

 cated in the accompanying 

 schematic figure (Fig. 237). 

 Within the brain itself there 

 are certain points at the an- 

 gles and hollows of the differ- 

 ent parts of the brain at which 

 the subarachnoidal space 

 is much enlarged, forming 

 the so-called cisternae, which 

 are in communication one 

 with another by means of the 

 less conspicuous canals (see 

 Fig. 238) . The whole system 

 is also in direct communica- 

 tion with the ventricles of 

 the brain on the one hand, 

 through the foramen of 



Magendie, the foramina of Luschka, and perhaps at other places, 

 and on the other hand, along the cranial and spinal nerves it is 

 continued outward in the tissue spaces of the sheaths of these nerves. 

 The Pacchionian bodies constitute also a peculiar feature of the sub- 

 arachnoidal space. These bodies occur in numbers that vary with 

 the individual and with age, and are found along the sinuses, 

 especially the superior longitudinal sinus. Each body is a minute, 



Fig. 237. Diagram to show the connec- 

 tion of the subarachnoidal space in the brain 

 and the cord. 



