THE CONTRACTILE TISSUES, 169 
in general. It is important to bear in mind that ciliary action 
may go on in the cells of a tissue completely isolated from the 
animal to which it belongs, and though influenced, as just ex- 
plained, by the surroundings, that the movement is essentially 
automatic, that is, independent of any special stimulus, in which 
respect it differs a good deal from voluntary muscle, which 
usually, if not always, contracts only when stimulated. 
The lines along which the evolution of the contractile tissues 
has proceeded from the indefinite outflowings and withdraw- 
als of the substance of Ameeba up to the highly specialized 
movements of a striped muscle-cell are not all clearly marked 
out; but even the few facts mentioned above suffice to show 
gradation, intermediate forms. A similar law is involved in 
the muscular contractility manifested by cells with other func- 
tions. The automatic (self-originated, independent largely of 
a stimulus) rhythm suggestive of ciliary movement, more 
manifest in the earlier developed smooth muscle than in the 
voluntary striped muscle of higher vertebrates, indicating 
further by the regularity with which certain organs act in 
which this smooth muscular tissue is predominant, a relation- 
ship to ciliary movement 
something in common as to 
origin—in a word, an evo- 
lution. And if this be 
borne in mind, we believe 
many facts will appear in 
anew light, and be invested 
with a breadth of meaning 
they would not otherwise 
possess, 
The Irritability of Muscle 
and Nerve.—An animal, as 
a frog, deprived of its 
brain, will remain motion- 
less till its tissues have 
died, unless the animal be Fia@. 161.—Nodes of Ranvier and lines of Fromann 
in some way stimulated. If toi with diver ittate, “B: Nerveiber from 
a muscle be isolated from the sclatic nerve of a full-grown rabbit. A, node 
z of Ranvier ; M, medullary substance rendered 
the body with the nerve to Enaestarent byte subioniot elycerine, Y.an 
which it belongs, it will are very distinct near the node, ‘The lines are 
é : less marked at a distance from the node. 
also remain passive; but, 
if an electric current be passed into it, if it be pricked, pinched, 
touched with a hot body or with certain chemical reagents, 
