^0$ Analogy between Vegetables and Animals^ 



afford support or nourishment ; it is, therefore, received by 

 the extremities of the veins, and brought back by them to 

 the heart, to acquire the addition of fresh materials from 

 the food, to be again ventilated in the lungs, and again to 

 be distributed all over the body. Such is the circulation of 

 the blood, as it is exhibited in man and other animals of the* 

 Mammalia class, in which the heart is of the most complex cha- 

 racter, and consists ol four distinct cavities. In Fishes, the 

 Construction of the heart is much more simple, and possesses 

 only two cavities. As we descend in the scale of animal life, 

 we find the heart presenting a progressively greater simplicity 

 of organisation ; for, in the more perfect order of Fermes, the 

 only appearance of a heart that can be seen is one or two 

 small dilatation& appearing on the branches which unite the 

 abdominal and dorsal bloodvessels together. As we descend 

 another step, we discover that there are some animals, desti- 

 tute of a heart altogether. This is the case with some of the 

 genera of the Mollusca, and the higher order of Zoophytes. 

 In these animals, the circulation is carried on by only Pwo 

 distinct sets of vessels. By one set, the blood is distributed, 

 all over the body ; by the other, it is brought back to the point, 

 whence it first set out. In animals still less perfect than 

 Zoophytes, as the Infusoria, and the inhabitants of sponges, 

 there is no circulating apparatus whatever to be found. 

 f-Jn the higher order of plants, the sap, like the blood 

 of animals, to which it may be considered analogous,, is 

 perpetually flowing in a complete circle. We perceive it 

 carried up by the vegetable lymphatics or veins, irpxa, 

 the roots, along the stem and branches, to the leaves*' ^r' Iff 

 the. upper surface of each leaf, it is circulated through thoo^ 

 Sands of minute vessels ; where it is acted on by the air 

 and light. As soon as it has undergone its proper changes^ 

 it is received by the capillary branches of another set of 

 yessels, which are ramified upon the lower surface of the 

 Jeaf. These reconduct it along the branches and stem, to- 

 wards the root. In its progress downwards, it supplies abun- 

 dant matter for the nourishment and increase of the plant, and 

 the expenditure of all the different secretions. What remains 

 after all these purposes have been answered returns to, and 

 oozes through, the spongioles ; to be again absorbed along 

 !^ith the other substances from the soil, to be again circulated 

 through the stem and branches, and again subjected to a 

 repetition of all the same processes. This description will be 

 sufficient to show the features of resemblance between the course 

 of the sap in vegetables, and that of the blood in animals. 



In the vegetable system, we see the crude sap, or lymph, 

 conveyed from the root to the leaves, in the same manner as 



