Vegetable Physiology. 19 



ference, since some animals so nearly resemble plants as to 

 make it almost impossible to detect the line of demarcation. 

 But in general, and among the higher orders of animals, the 

 distinction is so clear as to require no demonstration. An 

 animal may be defined as a living body possessed of sensibility 

 and voluntary motion. Locomotion, or the power of changing 

 from place to place, is not, however, essential, since ani- 

 mals exist which are fixed to one spot. But all animals have 

 a stomach, or internal cavity, into which is received matter 

 from without, to be elaborated into a nutritive substance. 

 This stomach does not occur in plants. Animals are also pos- 

 sessed of a nervous system and muscular fibres capable of 

 contraction, together with the power of selecting the matter 

 which is to afford them nutriment. Thus their food is com- 

 posed of many kinds of sanimal and vegetable substances, for 

 the digestion of which the organs which perform that office are 

 variously modified and arranged. But plants, being unable to 

 move from a particular spot, and to search for their food, are 

 nourished by the substances which surround them, and absorb, ' 

 by their surfaces, air, water, and many matters which these 

 contain in solution. Thus, from the little diversity of their 

 food, results the great uniformity in structure and purpose of 

 their organs. The parts of the animal body are also possessed 

 of mobility, and do not preserve a fixed position, while those 

 of the vegetable are nearly stationary, so that the impulse 

 which causes the distribution and motion of the nutritious 

 fluids in the former, is internal, while in the latter it acts from 

 without, and is not connected with the organization. Thus the 

 operation of the digestive organs, which in man forms chyle, is 

 independent of the conditions which are essential to the analo- 

 gous operation in plants, such as heat, moisture, and evapora- 

 tion. The chemical composition of both is the same, but the 

 elements are differently combined. The organs of sensation 

 and of motion, nerves properly so called, and muscles, are 

 entirely wanting in vegetables. They possess no heart or 

 other great organ of circulation, and have no vessels resem- 

 bling arteries or veins. There are some plants, notwithstand- 

 ing, which would seem to form exceptions to the general prin- 

 ciple that they lack sensibility and voluntary motion. The 



