370 THE BRAIN 



which projects downwards into the vallecula, and is called 

 the inferior vermis. 



When the inspection of specimens from which the mem- 

 branes have been removed is finished, the dissector should 

 turn to the examination of the membranes which still cover 

 his own specimen; they are the arachnoid and the pia mater. 



Arachnoidea Encephali. The arachnoid forms an inter- 

 mediate covering for the brain. It is placed between 

 the dura mater and the pia mater; it is directly continuous 

 with the arachnoid of the spinal medulla ; and it is con- 

 nected with the dura mater and the pia mater along the 

 nerve roots and along the blood vessels of the brain. It 

 is an exceedingly thin and delicate membrane, which can 

 be seen best on the base of the brain, because, 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 lower surface of the brain which lies anterior to the 

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

 be seen passing through the arachnoid ; whilst, anteriorly, 

 immediately 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 connect 

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

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

 delicate sponge-like interlacement between them. Where 

 the arachnoid passes over the summit of a cerebral gyrus, 

 and is consequently closely applied to the subjacent pia mater, 

 the meshwork is so close and the trabeculse 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 

 angular spaces exist between the arachnoid and the pia mater. 

 In those spaces the subarachnoid tissue can be studied, and 

 it will be seen that the spaces serve as communicating 



