MEMBRANES AND BLOOD VESSELS 439 



fissura lateralis (O.T. Sylvian fissure), which intervenes between 

 the pointed and projecting extremity of the temporal lobe and 

 the frontal lobe of the cerebrum ; whilst in the median plane, 

 anteriorly, the longitudinal fissure will be seen between the 

 frontal portions of the cerebral hemispheres. On either side 

 of the longitudinal fissure, and separated from it by a narrow 

 gyrus, the olfactory tract and bulb may be recognised. 



MEMBRANES AND BLOOD VESSELS OF 

 THE BRAIN. 



Arachnoidea Encephali. The arachnoid forms the inter- 

 mediate covering of the brain. It is placed between the dura 

 mater and the pia mater, and is directly continuous with the 

 arachnoid of the spinal medulla. It is an exceedingly thin 

 and delicate membrane, which can be seen best on the base 

 of the brain, as in that locality it is not so closely applied to 

 the pia mater as elsewhere. Unlike the pia mater it does 

 not (except in the case of the longitudinal and the lateral 

 fissures) dip into the sulci or fissures on the surface of the 

 cerebrum and cerebellum. It bridges over the inequalities on 

 the surface of the brain and it is spread out in the form of a 

 very distinct sheet over the medulla oblongata, the pons, and 

 the hollow on the base of the brain which lies anterior to the 

 pons. The cut ends of several of the cerebral nerves will be 

 seen passing through this sheet ; whilst, anteriorly, immedi- 

 ately to the lateral side of the optic nerve, the internal carotid 

 artery will be noticed piercing it. 



Cavum Subarachnoideale. The interval between the arach- 

 noid and the pia mater receives the name of the subarachnoid 

 space. It contains the subarachnoid fluid, and is broken up 

 by a meshwork of fine filaments and trabeculae, which connects 

 the two bounding membranes (viz., the arachnoid and the pia 

 mater) in the most intimate manner, and form a delicate 

 sponge-like interlacement between them. Where the arach- 

 noid passes over the summit of a cerebral gyrus, and is con- 

 sequently closely applied to the subjacent pia mater, the 

 meshwork is so close and the trabeculae so short that the 

 two membranes cannot be separated from each other. To 

 the dissector they appear to form a single lamina. In the 

 intervals between the rounded margins of adjacent gyri distinct 



