THE NERVOUS SYSTEM. 285 



Falx cerebri is an arched or sickle-shaped process received 

 into the longitudinal fissure. It contains in its upper and lower 

 margins the superior and inferior longitudinal sinuses. 



Tentorium cerebelli is a lamina, arched across, between the 

 superior borders of the petrous portion of temporal on either side, 

 the anterior and posterior clinoid processes in front, and the 

 transverse ridge of the occipital behind, for the support of the 

 posterior lobes of the cerebrum. It incloses the superior petrosal 

 and lateral sinuses. 



Falx cerebelli is a small median partition descending from 

 the tentorium to the foramen magnum, and separating the lateral 

 lobes of the cerebellum. 



The arachnoid is a thin, transparent, delicate membrane con- 

 sisting of fibrous and elastic tissue, and situated between the 

 dura and pia mater. From the former it is separated by the 

 subdural space; from the latter, by the subarachnoid space. 



Unlike the pia mater, it does not dip into the sulci on the 

 brain surface, except the sylvian and great longitudinal fissures. 



The subarachnoid space contains cerebro-spinal fluid, and 

 communicates through certain foramina with the cavities of the 

 brain. The space is much broken up by loose connective tissue, 

 which connects it to the pia mater. 



The three expanded portions of the subarachnoid space are 

 the cisterna magna, cisterna pontis, and cisterna basalis. 



The cislerna magna is placed over the roof of the lower 

 portion of the fourth ventricle, and communicates with the 

 fourth ventricle by the foramen of Majendie and two smaller 

 foramina : those of Keycs and Eetzius. 



The cisterna pontis is placed in front of the pons Varolii. 



The cisterna basalis incloses the circle of Willis. 



The Pacchionian bodies are projections of the arachnoid 

 into, but not through, the dura mater, and are most numerous 

 along the great longitudinal sinus. They make impressions on 

 the under surface of the calvarium, from which, however, they 

 are separated by a thin layer of dura mater. Their function is to 

 allow the passing of fluid from the subarachnoid space into the 

 brain-sinuses when the blood-pressure in the sinuses is lower than 

 in the subarachnoid space. 



Pia mater, the investing membrane of the brain, is composed 

 of a net- work of blood-vessels derived from the vertebral and 

 internal carotid arteries, held together by delicate connective 

 tissue. It dips into the sulci, and is prolonged into the ven- 

 tricles, forming the velum interpositum, or tela choroidea supe- 

 rior, and the tela choroidea inferior. The former covers the 

 third ventricle and extends into the lateral ventricles, carrying 



