148 PHILOSOPHY OF ZOOLOGY. 
dered by many authors as undetermined. Mr Car iyLr, 
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. 
