148 PHILOSOPHY OF ZOOLOGY. 



dered by many authors as undetermined. Mr CARLTLK, 

 however, seems disposed to consider the question as of 

 easy solution, and states the result of his investigations 

 in the following terms : " The terminal extremities of 

 nerves have been usually considered of unlimited extension. 

 By accurate dissection, however, and the aid of magnifying 

 glasses, the extreme fibrils of nerves are easily traced, as far 

 as their sensible properties and their continuity extend. 

 The fibrils cease to be subdivided, whilst perfectly visible 

 to the naked eye, in the voluntary muscles of large animals ; 

 and the spaces they occupy upon superficies where they 

 seem to end, leave a remarkable excess of parts unoccupied 

 by those fibrils. The extreme fibrils of nerves lose their 

 opacity ; the medullary substance appears soft and trans- 

 parent; the enveloping membrane becomes pellucid ; and 

 the whole fibril is destitute of the tenacity necessary to pre- 

 serve its own distinctness. It seems to be diffused or min- 

 gled with the substances in which it ends. Thus the ulti- 

 mate terminations of nerves for volition, and ordinary sen- 

 sation, appear to be in the reticular membrane, the com- 

 mon covering of all the different substances in an animal 

 body, and the connecting medium of all dissimilar parts *.*" 

 When it is considered that the nerves of sensation and vo- 

 lition exercise functions so very different from each other, 

 they may be expected to exhibit corresponding differences 

 in their connections and terminations. To detect these, the 

 scalpel and the microscope are necessary, under the guidance 

 of a mind habituated to observation, and cautious in its in- 

 ductions* 



The differences which may be observed in the nervous 

 system of the vertebral animals are numerous, and have 

 long occupied the attention of physiologists. But the ob- 



Phil. Trans. 1805, p. 9. 



