

THE ARACHNOID 



877 



In front, the cisterna interpeduncularis extends forward across the optic chiasma, 

 forming the cisterna chiasmatis, and on to the upper surface of the corpus callosum, 

 for the arachnoid stretches across from one cerebral hemisphere to the other immedi- 

 ately beneath the free border of the falx cerebri, and thus leaves a space in which 

 the anterior cerebral arteries are contained. The cisterna fossae cerebri lateralis is 

 formed in front of either temporal lobe by the arachnoid bridging across the lateral 

 fissure. This cavity contains the middle cerebral artery. The cisterna venas 

 magnae cerebri occupies the interval between the splenium of the corpus callosum 

 and the superior surface of the cerebellum ; it extends between the layers of the tela 

 chorioidea of the third ventricle and contains the great cerebral vein. 



Optic chiasma 





Cisterna interpeduncularis 



Fourth ventricle 

 Cisterna pontis 



Cisterna 



cerebellomedullaris 

 FIG. 768. Diagram showing the positions of the three principal subarachnoid cistern. 



The subarachnoid cavity communicates with the general ventricular cavity 

 of the brain by three openings; one, the foramen of Majendie, is in the middle line 

 at the inferior part of the roof of the fourth ventricle; the other two are at the 

 extremities of the lateral recesses of that ventricle, behind the upper roots of the 

 glossopharyngeal nerves and are known as the foramina of Luschka. It is still some- 

 what uncertain whether these foramina are actual openings or merely modified areas 

 of the inferior velum which permit the passage of the cerebrospinal fluid from the 

 ventricle into the subarachnoid spaces as through a permeable membrane. 



The spinal part of the subarachnoid cavity is a very wide interval, and is the 

 largest at the lower part of the vertebral canal, where the arachnoid encloses 

 the nerves which form the cauda equina. Above, it is continuous with the cranial 

 subarachnoid cavity; below, it ends at the level of the lower border of the second 

 sacral vertebra. It is partially divided by a longitudinal septum, the subarachnoid 

 septum, which connects the arachnoid with the pia mater opposite the posterior 

 median sulcus of the medulla spinalis, and forms a partition, incomplete and cribri- 

 form above, but more perfect in the thoracic region. The spinal subarachnoid 

 cavity is further subdivided by the ligamentum denticulatum, which will be described 

 with the pia mater. 



The cerebrospinal fluid is a clear limpid fluid, having a saltish taste, and a slightly alkaline 

 reaction. According to Lassaigne, it consists of 98.5 parts of water, the remaining 1.5 per cent, 

 being solid matters, animal and saline. It varies in quantity, being most abundant in old persons, 

 and is quickly secreted. 



