THE NERVOUS SYSTEM. 365 



LESSON IX. 

 THE NERVOUS SYSTEM AND ORGANS OF SENSE. 



1. THE NERVOUS SYSTEM may perhaps be considered the 

 primary and most important of all the systems of parts of 

 which the body is composed, because it dominates and 

 directs, as it were, the actions of the other parts. Moreover, 

 sometimes at least, it serves as a criterion in settling dis- 

 puted homologies of structures which belong to other systems. 

 Thus, the question as to what bone in one animal answers to 

 what bone in another animal is often determined, as in the 

 case of some cranial bones, by the several relations of such 

 bones to a certain nerve ; and the same kind of test may 

 not improbably serve to determine many muscular homo- 

 logies also. 



2. The PRIMARY STRUCTURES of which the nervous system 

 is composed (i.e. nerve-fibres and ganglionic corpuscles] have 

 been described in the " Elementary Physiology," Lesson 

 XII. " 1 6 and 19, and the nervous system as a whole has 

 also been sketched in Lesson XL of the same work. 

 Therein have been duly set forth its main component parts 

 the brain and spinal marrow (or cerebro-spinal axis), to- 

 gether with the membranes which invest them as also the 

 nerves issuing from such parts, including those which are 

 spoken of as the sympathetic system. 



3. Here we must recapitulate so far as to state that the 

 solid structures (skull and neural vertebral canal) which pro- 

 tect the cerebro-spinal axis are lined by a dense membrane 

 the dura mater y while the cerebro-spinal axis itself is closely 

 invested by a delicate membrane the pia mater. Inter- 

 posed between the two is a double very delicate epithelial 

 layer (called the arachnoid}, forming a shut sac (as the peri- 

 toneum forms a shut sac) and containing the arachnoid 

 fluid. 



