876 NEUROLOGY 





Structure. The spinal dura mater resembles in structure the meningeal or supporting layer 

 of the cranial dura mater, consisting of white fibrous and elastic tissue arranged in banda or 

 lamellae which, for the most part, are parallel with one another and have a longitudinal arrange- 

 ment. Its internal surface is smooth and covered by a layer of mesothelium. It is sparingly 

 supplied with bloodvessels, and a few nerves have been traced into it. 



The Arachnoid. 



The arachnoid is a delicate membrane enveloping the brain and medulla spinalis 

 and lying between the pia mater internally and the dura mater externally; it is 

 separated from the pia mater by the subarachnoid cavity, which is filled with 

 cerebrospinal fluid. 



The Cranial Part (arachnoidea encephali) of the arachnoid invests the brain 

 loosely, and does not dip into the sulci between the gyri, nor into the fissures, with 

 the exception of the longitudinal. On the upper surface of the brain the arachnoid 

 is thin and transparent; at the base it is thicker, and slightly opaque toward the 

 central part, where it extends across between the two temporal lobes in front 

 of the pons, so as to leave a considerable interval between it and the brain. 



The Spinal Part (arachnoidea spinalis} of the arachnoid is a thin, delicate, tubular 

 membrane loosely investing the medulla spinalis. Above, it is continuous with 

 the cranial arachnoid ; below, it widens out and invests the cauda equina and the 

 nerves proceeding from it. It is separated from the dura mater by the subdural 

 space, but here and there this space te traversed by isolated connective-tissue 

 trabeculse, which are most numerous on the posterior surface of the medulla spinalis. 



The arachnoid surrounds the cranial and spinal nerves, and encloses them 

 in loose sheaths as far as their points of exit from the skull and vertebral canal. 



Structure. The arachnoid consists of bundles of white fibrous and elastic tissue intimately 

 blended together. Its outer surface is covered with a layer of low cuboidal mesothelium. The 

 inner surface and the trabeculse are likewise covered by a somewhat low type of cuboidal meso- 

 thelium which in places are flattened to a pavement type. Vessels of considerable size, but few 

 in number, and, according to Bochdalek, a rich plexus of nerves derived from the motor root 

 of the trigeminal, the facial, and the accessory nerves, are found in the arachnoid. 



The Subarachnoid Cavity (cavum subarachnoideale; subarachnoid space) is the 

 interval between the arachnoid and pia mater. It is occupied by a spongy tissue 

 consisting of trabeculse of delicate connective tissue, and intercommunicating 

 channels in which the subarachnoid fluid is contained. This cavity is small on the 

 surface of the hemispheres of the brain; on the summit of each gyrus the pia mater 

 and the arachnoid are in close contact; but in the sulci between the gyri, triangular 

 spaces are left, in which the subarachnoid trabecular tissue is found, for the pia 

 mater dips into the sulci, whereas the arachnoid bridges across them from gyrus to 

 gyrus. At certain parts of the base of the brain, the arachnoid is separated from the 

 pia mater by wide intervals, which communicate freely with each other and are 

 named subarachnoid cisternae; in these the subarachnoid tissue is less abundant. 



Subarachnoid Cisternae (cisternae subarachnoidales) (Fig. 768). The cisterna 

 cerebellomedullaris (cisterna magna) is triangular on sagittal section, and results 

 from the arachnoid bridging over the interval between the medulla oblongata 

 and the under surfaces of the hemispheres of the cerebellum; it is continuous 

 with the subarachnoid cavity of the medulla spinalis at the level of the foramen 

 magnum. The cisterna pontis is a considerable space on the ventral aspect of the 

 pons. It contains the basilar artery, and is continuous behind with the subarach- 

 noid cavity of the medulla spinalis, and with the cisterna cerebellomedullaris; and 

 in front of the pons with the cisterna interpeduncularis. The cisterna interpeduncu- 

 laris (cisterna basalis) is a wide cavity where the arachnoid extends across between 

 the two temporal lobes. It encloses the cerebral peduncles and the structures 

 contained in the interpeduncular fossa, and contains the arterial circle of Willis. 



