CIRCULATION. 97 



Second cavity with thick and fleshy walls named the right 

 ventricle. The right ventricle, in contracting powerfully 

 upon itself, drives the blood through the pulmonary artery 

 and its innumerable ramifications into the lungs. There 

 the blood is brought into immediate contact with the air 

 absorbed during respiration, and is oxygenated anew, losing 

 a part of its carbon, which forms the carbonic acid which 

 we expire. It is returned afterwards from the lungs by 

 the pulmonary veins to the left auricle, and from thence it 

 is poured into the left ventricle, which, by its contractions, 

 drives the blood through the large artery named the aorta, 

 whose ramifications again carry it to all parts of the body. 

 The nutrient fluid which circulates through the organic 

 tissues of plants, and which is called sap, exercises the 

 same function in the vegetable that the blood does in the 

 animal economy. Plants, however, possess no proper 

 vessels within which -a true circulation is maintained by 

 the muscular action of a central propelling organ or heart, 

 and the sap of plants is not confined like the blood of ani- 

 mals to one set of vessels, for owing to the way in which 

 the vascular and cellular tissues of plants are interwoven 

 with each other, and the general permeability of all the 

 organs, a general transfusion of the sap takes place from 

 cell to cell, endosmotically, and in every direction, so that 

 the process is in some respects one of distribution as well 

 as of circulation. This is particularly the case in the em- 

 bryos of flowering plants during the early stages of their 

 growth, whilst their structure continues wholly cellular, 

 and in those cryptogamous plants which permanently re- 

 main in this low condition of development. But as soon 

 as the vital action of the embryo of flowering plants com- 

 mences, and woody fibre and vascular tissue begin to 

 appear in its expanding organs, another force comes into 

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