618 



CIRCULATION OF BLOOD AND LYMPH. 



as the superior cerebral vein. A point of physiological interest is 

 that the venous sinuses and their points of emergence from the skull 

 are by their structure well protected from closure by compression. 

 The Meningeal Spaces and Cerebrospinal Liquid. — The general 

 arrangement of the meningeal membranes, and particularly of the 

 meningeal spaces, is important in connection with the mechanics of 

 the brain circulation. In the skull the dura mater adheres to the bone, 

 the pia mater invests closely the surface of the brain, while between 

 lies the arachnoid (Fig. 257). The capillary space between the arach- 

 noid 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. 258). 

 Within the brain itself there 

 are certain points at the an- 

 gles and hollows of the d'.ffer- 

 ent parts of the brain at which 

 the subarachnoidal space 

 is much enlarged, forming 

 the so-called cisternse, which 

 are in communication one 

 with another by means of the 

 less conspicuous canals (see 

 Fig. 259). 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 pecuUar 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, 



Fig. 258.— Dia, 

 tion of the subaracl 

 and the cord. 



to show the connec- 

 oidal space in the brain 



