660 THE CENTRAL AXIS OF THE NERVOUS SYSTEM. 



articulation, it widens again ; after which it rapidly lessens, and altogether 

 disappears towards the fourth or fifth coccygeal vertebra. The lumbo- 

 eacral dilatation coincides with the expansion the cord shows at this point, 

 and with the enormous volume of the nerves lying beside it. 



2. The Cranial Cavity. (Figs. 22, 23, 175.) 



This is a very irregular oval box, whose walls are formed by the frontal, 

 parietal, occipital, ethmoidal, and temporal bones. 



It presents for consideration four planes and two extremities. 



The superior plane offers on the middle line, and towards its superior 

 third, the parietal protuberance, the two lateral crests of which concur 

 with that eminence in dividing the cranial cavity into two compartments : 

 one posterior, destined to contain the cerebellum; the other anterior, 

 incomparably larger, lodging the cerebral hemispheres, and divided by the 

 single rudimentary crest which begins at the falciform eminence, and joins 

 the crista galli, into two lateral sections one for each hemisphere. Here- 

 after we will 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 

 ala of the sphenoid bone. 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 median line, the basilar channel, into which the greater portion of the 

 encephalic isthmus 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 

 niastoid lobule, or inferior lobe of the brain. 



The posterior extremity of the cranial cavity presents 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 

 aspect of the transverse plate of that bone. 



THE ENVELOPES OF THE CEREBRO-SPINAL AXIS. 



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

 it from the walls of the bony cavity inclosing 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 



