THE CEBEBBO-SPINAL AXIS. 659 



It is worthy of remark that the properties of the nervous system, which 

 act in so important a manner on the organs of vegetative Ufe, have no 

 direct influence on nutrition itself. Destruction of the nerves in a certain 

 region will certainly derange the nutrition of its tissues, in consequence of 

 the paralysis of the vessels, but it is not destroyed. There is an immense 

 category of organised beings— vegetables, for instance— in which nutrition 

 is very active, and in which there is no nervous system. So that the 

 property which determines the essential phenomena of nutrition is iadepen- 

 dent of nervous action : it is an attribute of living matter. 



SECOND SECTION. 



The Central Axis of the Nervous System. 



The cerebrospinal axis is resolved, as we have said, into two principal 

 sections — the spinal cord and the encephalon. We will study these two 

 portions in succession, the spinal cord first, in order to facilitate our de- 

 scription, although that organ only holds the second place in a physiological 

 point of view. The protective parts of these two apparatus will, however, be 

 examined before we proceed further. 



CHAPTEE I. 



the enveloping and protecting parts op the ceeebro-spinal 



AXIS. 



The cerebro-spinal apparatus is lodged, as we already know, 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 dura mater, arachnoid, and pia mater. 



THE BONY CASE THAT CONTAINS THE CEEEBKO-SPINAL AXIS. 



A knowledge of the bones which enter into the composition of this pro- 

 tective 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 spinal canal, and m 

 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 movements 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 the commencenunt 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 offers its 

 smallest diameter, but on leaving this portion, and asfarasthelumbo-sacral 



