106 THE STOCK owner's ADVISER. 



Thej usually aocompany the arteries; but there are exceptions 

 to this rule. The veins have thinner walls than the arteries; 

 their inner coat closely resembles that of the arteries. The mid- 

 dle coat is thinner and less muscular, and contains more white, 

 fibrous tissue than the middle coat of the arteries. The external 

 coat consists of connective tissue and elastic fibers. A feature 

 peculiar to the veins is the existence of valves of various con- 

 struction, which prevent the blood from returning upon its 

 course, and assist in impelling it toward the heart. The veins 

 of the extremities, neck, and scalp have numerous valves, which 

 are absent for the most part in the deep veins of the abdomen, 

 chest, and cranium. The dark, purplish blood of the veins 

 empties into the heart again at the right auricle; after passing 

 through the auriculo-ventricular opening to the right ventricle, 

 it passes through the pulmonary artery to the lungs, where the 

 impure, dark blood is purified and changed to a scarlet color, 

 and sent again on its round. 



The lymphatic system, connected with the blood vascular 

 system, consists of a series of tubes, which absorb and convey 

 to the blood certain fluids, lymph and chyle. They take their 

 origin in every tissue of the body supplied with blood; they 

 carry back into the vascular system any excess of the plasma of 

 the blood which has transuded from the capillaries, and is not 

 required for the nutrition of the tissues. 



The lym])hatics unite to form two large trunks, the thoracic 

 duct and the right lymphatic vein, botli of which enter the 

 venous system near the heart. Delicate in structure and trans- 

 parent, they are j^resent in nearly every tissue, and although 

 more numerous than the blood vessels, their collective capacity 

 is probably not greater. 



RESPIRATORY ORGANS. 



Respiration is carried on by the lungs, bronchial tubes, 

 trachea, larynx, and the nasal chambers, with the assistance of 

 the inspiratory muscles and the respiratory nerves. The lungs 



