748 THE CENTRAL AXIS OF THE NERVOUS SYSTEM. 



destined to contain the cerebellum ; the other anterior, incomparably larger, 

 lodging the cerebral hemispheres, and divided by the single rudimentary crest 

 which iDCgins at the falciform process, and joins the crista galli, into two lateral 

 sections — one for each hemisphere. Hereafter we shall see that the folds of the 

 dura mater are attached to this parietal protuberance, and to the ridges detached 

 from it, thus rendering much more perfect the partitioning of the cranial cavity. 



On the lateral planes there is also noticed the division into a cerebellar and 

 cerebral compartment, due to the lateral crests of the falciform process, which are 

 prolonged obliquely to near the sphenoid bone ; the first section is formed by the 

 occipital and the inner face of the petrous bone ; the second by the squamous 

 portion of the temporal, the frontal, and the great wing of the sphenoid l)one. 

 Both are concave, and marked by digital impressions, as they also are on the 

 superior plane. 



The inferior plane, very irregular, offers from behind forward : 1. On the 

 middle line, the basilar channel, into which the greater portion of the medulla is 

 received ; the pituitary fossa, made deeper by a circular fold of the dura mater, 

 and lodging the gland of that name ; the optic fossa, where the chiasma of the 

 optic nerves is situated. 2. On the sides, the foramen lacerum, partly closed by 

 cartilaginous substance, and by the dura mater ; the cavernous sinuses and 

 maxillary fissures, outside which is remarked a deep and wide digital impression 

 for the reception of the mastoid or inferior lobe of the brain. 



The posterior extremity of the cranial cavity shows the occipital foramen, by 

 means of which this cavity communicates with the spinal canal. 



The anterior extremity offers, in the median plane, the crista galli process, or 

 superior border of the perpendicular plate of the ethmoid bone ; on the sides, 

 the two ethmoidal fossae —deep depressions containing the olfactory lobes, and at 

 the bottom of which is observed the cribriform plate of that bone. 



The Envelopes of the Cerebro-spinal Axis. 



Preparation. — In order to study the cerebro-spinal axis, the same preparation as for the 

 spinal cord — to be alluded to hereafter - should be followed. When the centres are freed from 

 their bony covering, the membranes may be incised, dissected, and separated from each other 

 — the arrangement of the sub-arachnoid spaces being rendered apparent by insufflation. 



The arrangement of the cranial dura mater can be studied by making an antero-posterior 

 section of the cranium beyond the middle line, and a transverse section in front of the internal 

 occipital protuberance. On the first is seen the falx cerebri and pituitary fold, and on the second 

 the tentorium cerebelli. 



The three membranes which cover the cerebro-spinal axis, and separate it 

 from the walls of the bony cavity enclosing it, are thus designated. Generally 

 termed meninges, and distinguished as external, middle, and internal meninge, 

 these membranes are better known as the dura mater, arachnoid, and pia mater 

 — names which will be employed in our description. 



The dura mater, or external meninge, is a strong fibrous membrane in contact 

 with the walls of the cranium and the spinal canal. 



The arachnoid, or middle meninge, is a tunic of a serous nature, which resolves 

 itself into two layers — an external, applied to the inner face of the dura mater ; 

 and an internal, spread, through the medium of the pia mater, over the cerebro- 

 spinal axis, from which it is again separated at a great number of points by a 

 particular fluid, the sub-arachnoid (or liquor cerel/ro-s/)inalis). 



The pia mater, or internal meninge, is the proper envelope of the central 



