18 PHYSIOLOGY. 



velopment is towards the formation of a stomach, for the internal 

 reception and digestion of food ; whilst the first processes of vegetable 

 evolution tend to the production of a leaf-like membrane, which, like 

 the permanent y/'OTaCi? of the lower classes of plants, absorbs nourish- 

 ment by its expanded surface only. The seed of the plant and the 

 egg of the animal consist principally of a store of nourishment pre- 

 pared by the parent for the supply of the germ, which is introduced 

 into the midst of it. In both instances, the first development of the 

 germ is into a membranous expansion, which absorbs the alimentary 

 materials with which it is in contact, and prepares it for the nourish- 

 ment of the embryonic structure. In plants, this membranous expan- 

 sion absorbs by its outer surface only, which is applied to the albumen 

 of the seed. In animals, the expansion is developed in such a man- 

 ner, that it surrounds the albumen, inclosing it in a sac, of which 

 the inner surface only is concerned in absorption, and becomes the 

 temporary stomach of the embryonic structure. 



The circulation is much more simple in plants than in animals, 

 and is, in them, never provided with a special organ for the distribu- 

 tion of the sap ; they have no heart. 



The function of respiratio7i has usually been enumerated among 

 the distinctive characteristics of animals and plants, it being supposed 

 that oxygen is absorbed and carbonic acid evolved by the former, 

 and a converse change effected in the surrounding air by the 

 latter. This, however, is not correct, the products and process of 

 respiration being the same in both ; whilst the absorption of carbonic 

 acid and elimination of oxygen which take place only in daylight, 

 is much more analogous to the digestion of animals. Lastly, plants 

 having only one mode of manifesting life, namely, by vegetation, do 

 not require manifold organs in addition to their roots, stem, and 

 leaves, and, with the exception of the organs of fructification, present 

 merely a repetition of perfectly similar parts, in all of which the 

 simple relation of branches to leaves is the same, and even the sexual 

 organs are evidently allied to the leaves, and in some cases are trans- 

 formed into them. In animals, on the contrary, the reciprocal action 

 of circulation, respiration, and the nervous system is actually neces- 

 sary to life. The respiratory movements are dependent on nervous 

 influence ; but the nerves do not exert this influence unless supplied 

 with blood which has been aerated in the lungs ; and the blood again 

 is not sent to the different organs, and therefore not to the nerves, 

 unless the contractions of the heart are performed ; while the heart 

 in its turn is dependent on the influence of arterial blood and the 

 nerves. The brain, heart, and lungs, therefore, may be likened to 

 the main wheels of the animal machine, each acting on the other, 

 and all set in motion by the change of material which takes place in 

 respiration. 



