THE VITAL AND PHYSICAL FORCES. 
747 
forces which that structure possessed. We are, then, to regard the nervous, elec- 
trical, and other stimuli, under whose influence the muscular force is called forth, 
less as the immediate sources of that force, than as furnishing the conditions under 
which the vital force acting through the muscle is converted into the mechanical 
force developed in its contraction. 
We do not yet know enough of the conditions under which ciliary movement takes 
place, to enable us to affirm that the production of mechanical motion through its 
means is in like manner the result of an expenditure of vital force ; but the considera- 
tions formerly adduced in regard to the relation of ciliary action to other vital mani- 
festations, together with the remarkable similarity between the influence of strychnia, 
opium, electric discharges, &c. upon the ciliary movement and upon muscular con- 
tractility, leave little room for doubt that what is true of the muscular force is true 
also of ciliary motion, and that it, too, is to be regarded as directly depending upon 
a conversion of vital force into meehanical motion. The continuance of motion in 
the cilia appears to be intimately related to changes taking place in the cells on whicli 
they are borne ; and its persistence after the detachment of these cells from the re- 
mainder of the body, like the persistence in the contractility of muscular fibre which 
has been completely isolated from all its connections, proves that we must look to 
forces existing in them, and not to influences derived from any other source, for the 
maintenance of this curious operation. 
Passing from these particular manifestations of vital force, which so remarkably 
indicate its relations to physical agencies, to those which, being concerned in the 
development and growth of organized structures, seem to have less in common with 
them, we shall fix our attention on the fundamental fact, that these Organizing forces 
(as we may conveniently designate them) are so completely dependent upon the con- 
tinual agency of Heat (and in some cases of Light also), that they may be considered 
as the manifestations of the action of heat upon organized fabrics. 
The necessity for this agency may be seen at every period of the life of organized 
beings of all kinds. In the lower tribes of animals, and in the entire vegetable king- 
dom, we trace this dependence in the precise relation between the vital activity of each 
individual, and the amount of heat which it receives from external sources. Every spe- 
cies is adapted to flourish within a certain range of temperature ; and that amount 
of heat which is most effective in sustaining the life of one species, may be injurious 
or even fatal to another. But within the range which is compatible with the mani- 
festation of its vital powers, we find that the relation is most constant between the 
temperature and the organizing force exhibited by each species. It is scarcely neces- 
sary to accumulate facts in support of a position so generally admitted; but the 
author particularly wishes to direct attention to the definiteness and exactness of this 
relation, and may instance the following facts as examples. 
1. According to Boussingault, the same annual plant, in arriving at its full de- 
velopment, and going through all the processes of flowering and maturation of its 
5 c 2 
