168 ANIMAL PHYSIOLOGY. 
sense. The non-striped muscle-cells are most abundant among 
the invertebrates, though found in the viscera and a few other 
parts of vertebrates. This form is plainly the simpler and 
more primitive. The voluntary muscles are of the striped 
variety in articulates and some other invertebrate groups and 
in all vertebrates; and there seems to be some relation between 
the size of the muscle-fiber and the functional power of the 
tissue—the finer they are and the better supplied with blood, 
two constant relations, the greater the contractility. 
Whether a single smooth muscle-cell, a striped fiber (cell), or 
a collection of the latter (muscle) be observed, the invariable _ 
result of contraction is a change of shape which is perfectly 
definite, the long diameter of the cell or muscle becoming 
shorter, and the short diameter longer. 
Ciliary Movements.—This subject has been already considered 
briefly in connection with some of the lower forms of life pre- 
sented for study. 
It is to be noted that there is a gradual replacement of this 
form of action by that of muscle as we, ascend the animal 
scale; it is, however,-retained even in the highest animals in 
the discharge of functions analogous to those it fulfills in the 
invertebrates. 
Thus, in Vorticella, we saw that the ciliary movements of 
the peristome caused currents that carried in all sorts of parti- 
cles, including food. Ina creature so high in the scale as the 
frog we find the alimentary tract ciliated ; and in man himself 
a portion of the respiratory tract is provided with ciliated cells 
concerned with assisting gaseous interchange, a matter of the 
highest importance to the well-being of the mammal. As be- 
fore indicated, ciliated cells are found in the female generative 
organs, where they play a part already explained. 
It is a matter of no little significance from an evolutionary 
point of view, that ciliated cells are more widely distributed in 
the foetus than in the adult human subject. 
As would be expected, the movements of cilia are affected 
by a variety of circumstances and reagents: thus, they are quick- 
ened by bile, acids, alkalies, alcohol, elevation of temperature 
up to about 40° C., etc.; retarded by cold, carbonic anhydride, " 
ether, chloroform, etc. 
In some cases their action may be arrested and re-estab- 
lished by treatment with reagents, or it may recommence with- 
out such assistance. All this seems to point to ciliary action as 
falling under the laws governing the movements of protoplasm 
