480 THE SENSORIAL FUNCTIONS. 



a consentaneousness of action in the operations 

 both of the sensorial and organic functions. 



It appears from this analysis, that the office of 

 the nervous fibres, whether afferent, efferent, or 

 commissural, is merely that of conductors of some 

 special but unknown agency, which may happen 

 to be excited at one point of their course, to their 

 terminal extremities ; while the grey neurine is the 

 seat of other unknown operations, by which the 

 impressions so conveyed are modified and com- 

 bined. 



The different portions of the nervous system, of 

 which I have now pointed out the structure and 

 the offices, are distinguishable from one another 

 only in the more highly developed forms of animal 

 organization. In all the inferior types of structure, 

 the ganglionic and the sensorial systems are closely 

 interwoven and intimately blended together, so that 

 it is extremely difficult to separate them, and as- 

 sign to each portion its respective function. What 

 adds to the difficulty in the case of the ordinary 

 nerves, is the circumstance that the afferent and 

 efferent nerves of both systems are collected to- 

 gether within the same membranous envelope, or 

 neiirilema, as it is termed, so as to form but a single 

 cord. In the inferior departments of the animal 

 kingdom, the evidence of sensation and volition are 

 often extremely ambiguous : for it is difficult to 

 discriminate those movements M'hich are really 

 consequent on volition, from those which are the 

 results of mere vital or reflex irritation, and are 

 unattended M-ith any degree of consciousness : and 

 the lower we descend in the scale of animals, the 

 more reason we find for suspecting the predomi- 



