THE ORGANS 23 



matters in solution or suspension, and innutritious dregs 

 or faeces. 



8. Circulatory Organs. — A system of minute tubes, 

 with very thin walls, termed capillaries, is distributed 

 through the whole organism except the epidermis and its 

 products, the epithelium, the cartilages, and the sub- 

 stance of the teeth. On all sides, these tubes pass into 

 others, which are called arteries and veins ; while 

 these, becoming larger and larger, at length open into the 

 heart, an organ which, as we have seen, is placed in the 

 thorax. During life, these tubes and the chambers of 

 the heart, with which chey are connected, are all full of 

 li(juid. which is, for the most part, that red fluid with 

 which we are all familiar as blood. 



The walls of the heart are muscular, and contract 

 rhythmically, or at regular intervals. By means of these 

 contractions the blood Avhich its cavities contain is driven 

 in jets out of these cavities, into the arteries, and thence 

 into the capillaries, whence it returns by the veins back 

 into the heart. 



This is the circulation of the blood. 



Now the fluid containing the dissolved nutritive 

 matters which are the result of the process of digestion, 

 traverses the very thin layer of soft and permeable 

 tissue which -separates tlie cavity of the alimentary canal 

 from the cavities of the innumerable capillary vessels 

 which lie in the walls of that canal, and so enters the 

 blood, with which those capillaries are filled. Whirled 

 away by the torrent of the circulation, the blood, thus 

 charged ^\-ith nutritive matter, enters the heart, and is 

 thence propelled into the organs of the body. To these 

 organs it supplies the nutriment with which it is charged ; 

 from them it takes their waste products, and, finally, 

 returns by the veins to the heart, loaded with usele-ss and 

 injurious excretions, which sooner or later take the form 

 of water, carbonic acid, and urea. 



9. Excretory Organs. — These excretionaiy matters 



