TEE CEBEBBO-SPINAL AXIS. 747 



SECOND SECTION. 



THE CENTRAL AXIS OP THE NERVOUS SYSTEM. 



The cerehro-spinal axis is resolved, as we have said, into two principal sections — 

 the spinal cord and the brain. "We will study these two portions in succession — 

 the spinal cord first, in order to facilitate description, although that organ only 

 holds the second place, from a physiological point of view. The protective parts 

 of these two apparatuses will, however, be examined before we proceed further. 



CHAPTER I. 



The Enveloping and Protecting Parts of the Cerebro-spinal Axis. 



The cerebro-spinal apparatus is lodged, as has been already mentioned, in a bony 

 case — ^the spinal canal — which is prolonged anteriorly by the cranial cavity ; but 

 it is protected more immediately by three envelopes, which have received the 

 names of chari mater, arachnoid, andjsia mater. 



The Bony Case containing the Cerebro-spinal Axis. 



A knowledge of the bones which enter into the formation of this protective 

 case cannot be acquired without also knowing the case itself ; so that we dispense 

 with its special study here. We will allude, however, to the succinct terms already 

 employed in describing the spijial canal, and in the same spirit of concision will 

 also describe what has hitherto been deferred — the cranial cavity. 



1. The Spinal Canal. 



This canal communicates, anteriorly, with the cavity of the cranium. Very 

 wide at the atlas — to receive the odontoid process, and allow those rotatory move- 

 ments of the head which prevent the medulla being injured — the spinal canal 

 suddenly contracts at the axis : it expands again at the end of the cervical and 

 commencement of the dorsal region, where the medulla presents a greater volume, 

 and the movements of the spine are very extensive. Towards the middle of the 

 back, the spinal canal has its smallest diameter ; but on leaving this portion, and 

 as far as the lumbo-sacral articulation, it widens again ; after which it rapidly 

 decreases, and altogether disappears towards the fourth or fifth coccygeal ver- 

 tebra. The lumbo-sacral 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. 41, 42). 



This is a very irregular, ovoid-shaped box, the walls of which 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, 



