140 THE POCKET ANATOMY 



posterior clinoid processes, superior edge of the petrous bone, 

 and behind to the margins of the groove for the lateral sinus. 

 The inner concave border is free posteriorly, and forms the 

 opening through which the crura cerebri and the posterior 

 cerebral arteries pass from the posterior into the middle cranial 

 fossa ; in front this border passes over the attached border, and 

 is attached to the anterior clinoid processes. The tentorium 

 is a tent-like process, highest above and in front, at its most 

 anterior point of attachment of the falx cerebri by which the 

 tentorium is suspended, and from this point descending on all 

 sides. In relation to it are the lateral, superior petrosal, and 

 straight sinuses. 



The falx cerebelli reaches vertically from the tentorium 

 to the foramen magnum, dividing the two hemispheres of 

 the cerebellum. It is attached posteriorly to the vertical 

 crest of the occiput, where it encloses the occipital sinus, and 

 below to each side of the foramen magnum. 



The PIA MATER consists of a delicate stroma supporting 

 bloodvessels, and closely invests the brain, dipping into the 

 sulci. At the transverse fissure it is prolonged into the lateral 

 ventricles and over the 3rd ventricle, pushing the endothelial 

 lining of those cavities in front of it, and forming the velum 

 interpositum and choroid plexuses of the lateral and 3rd ven- 

 tricles. It is prolonged over the roof of the 4th ventricle, 

 sending inwards two vascular fringes, the choroid plexuses of 

 that cavity. 



The ARACHNOID is a thin membrane lying outside the 

 pia mater, but is not so closely applied to the brain, as it 

 passes over the sulci without dipping into them. Between 

 the pia mater and the arachnoid is the subarachnoid space, 

 containing the subarachnoid fluid. The space between the 

 dura mater and arachnoid is now known as the subdural 

 space. 



The subarachnoid space is larger in some places than in 

 others. The arachnoid stretches across between the two 

 middle lobes at the base of the brain, forming the cisterna 

 basalis, which lies anterior to the pons and reaches as far 

 forward as the optic nerves. Beneath the cerebellum it forms 

 in a like manner the cisterna magna, by stretching between 

 the under surface of the cerebellum and the posterior surface 

 of the medulla. 



At the bottom of the longitudinal fissure the subarachnoid 

 space is also large. 



The subarachnoid space communicates with the cerebral 

 ventricles by the foramen of Majendie, an opening into the 



