1862.] on Motion in Plants and Animals, 435 



only very various in themselves, but obviously dependent on widely 

 different causes. They may, I think, be thus classified :* 



Motion due to extraneous causes. 



Molecular movement— the peculiar movement visible in the 

 minutest granules or molecules of matter when placed in a fluid which 

 is not too tenacious, and viewed by the highest powers of the microscope 

 — is an example of this kind of motion. It is due, in great part at 

 least, simply to vibration. 



Movements which are due to osmotic currents and other 

 hygroscopic changes. 



In the vegetable kingdom the expulsion of the seeds from the ripe 

 fruit of the spirting cucumber {Momordica Elaterium) may be selected 

 as a remarkable example. Although we cannot bring forward from 

 the animal kingdom such striking illustrations of osmotic force as 

 this one, yet there is every reason to believe that it is constantly 

 playing a most important part in the transference of matter. 



Movement due to the physical properties of a tissue. 



Elastic tissue plays an interesting and important part in various 

 movements of animals ; and this simply by means of its physical 

 property of elasticity which it possesses in perfection. This elastic 

 tissue is of course passive, until either extended or compressed by some 

 force — and this force is usually muscular action — when as the force is 

 remitted it recoils with proportionate power. Thus we commonly 

 consider its use in relation to muscular action. It may either assist 

 muscular action — be accessory to it, or it may be opposed to muscular 

 action — acting in an opposite direction. Elastic tissue is commonly 

 rendered active by stretching, sometimes however it is compressed. 

 As illustrations of these several modes and purposes of action may be 

 mentioned the functions of the Ligamentum Nuchce, of the spring and 

 ligament at the hinge of the shell of bivalves, of the elastic ligaments 

 attached to the claws of Felidce, and of the vocal cords. Again, 

 movements produced by the elastic recoil of stretched or distended 

 tissues in plants are common enough. To elasticity the bursting of 

 seed cases and anthers is without doubt to be in great measure 

 referred. 



Movements due to a structure possessing the vital property of 

 contractility — vital motion. 



By vital contractility is to be understood the power which certain 

 structures possess of altering their form, of approximating distant 

 parts by an inherent force. 



Examples of vital contractility in animals are found everywhere. 



♦ From the causes of motioa I exclude the changes wWch are involved in 

 nutrition ; for no attempt to review these could be brought within the scope of 

 this discourse. 



