DISSECTING MANUAL. 



Arachnoidea. This is a delicate membrane which dips into 

 the Sylvian and great longitudinal fissures only. The suba- 

 rachnoid space, between the arachnoidea and pia, contains 

 cerebro-spinal fluid; it communicates with the lateral ventri- 

 cles at the end of each descending horn, and with the fourth 

 ventricle at the foramen of Magendie and the tip of each lateral 

 recess. A fine meshwork connects it with the pia and devel- 

 ops interspaces (cisternse subarachnoidales) . [600] 



The Pacchionian bodies are small purplish protrusions from 

 the arachnoidea, and attached by a stalk, which invaginate 

 but do not pierce the dura. They occur chiefly in or be- 

 side the superior longitudinal sinus, occasionally in other 

 sinuses. [602] 



Pia Mater. This is a delicate vascular membrane envelop- 

 ing the brain closely and lining all sulci except the smaller 

 cerebellar ones. The tela choroidea inferior is the portion cov- 

 ering the ependymal roof of the fourth ventricle ; prolongations 

 from it (choroid plexuses) invaginate the ependyma. The 

 velum interpositum or tela choroidea superior is a triangular 

 double layer, or fold, passing through the transverse fissure, 

 between the fornix and the optic thalami and roof of the third 

 ventricle, to (apex) the foramina of Monro; prolongations 

 from it (choroid plexuses) invaginate the roof of the third 

 ventricle and, along its margins, the walls of the lateral ven- 

 tricles. [603] 



THE BRAIN. 



This consists of the medulla, pons, cerebellum, mid-brain, 

 and cerebrum ; it has a convex superior surface and an irreg- 

 ular inferior surface or base. 



MEDULLA. 



This extends from the foramen magnum to the pons, con- 

 tinuing the spinal cord; it is under an inch long and widest 

 above. It consists of a lower closed part and an upper open 



[26] 



