330 LYMPHATIC SYSTEM. 



which are so arranged as to permit the blood to flow in but one 

 direction that is, toward the heart. Many of these veins are so 

 situated with reference to muscles that when the muscles contract 

 the contiguous veins are compressed. This compression forces the 

 contained blood away from the points of pressure, and as the 

 closure of the valves prevents the blood from flowing backward, 

 it must go forward. 



Aspiration of the Thorax. At each inspiration the cavity 

 of the chest is enlarged and the pressure on its contents is dimin- 

 ished. One of the results of this inspiration is the inflowing of 

 air. Another result is the inflowing of blood into the venae cavse 

 and right auricle, for while the intrathoracic pressure is dimin- 

 ished, that upon the blood-vessels outside remains the same. A 

 similar tendency exists for the blood in the aorta to flow back into 

 the left ventricle, but this is prevented by the aortic valve. This 

 subject will be again discussed in connection with respiration. 



Force of Gravity. The force of gravity assists in the 

 return of the blood to the heart from the upper portions of the 

 body, but retards its return from the lower portions, so that as a 

 factor in aiding the circulation as a whole it may be ignored. This 

 force may, however, be utilized whenever for any reason there is 

 congestion in a part as, for instance, in a foot the seat of inflam- 

 mation. In such a case the elevation of the lower extremity 

 facilitates the flow of blood in the veins and proves beneficial. 

 Also, when by reason of an imperfect performance of its function 

 the heart fails to send enough blood to the brain, and fainting 

 occurs, relief will come more promptly if the patient is at once 

 placed on the back, with the head lower than the 'heart, thus 

 assisting that organ in sending blood to the anemic brain. 



LYMPHATIC SYSTEM. 



The lymphatic system is composed of lymphatic vessels, lymphatic 

 glands, and the cavities of the serous membranes. 



I/ymphatic Vessels. The larger lymphatic vessels struct- 

 urally are like the veins, being composed of three coats, the 

 middle coat containing both muscular and elastic fibers. Unlike 

 the veins, however, muscular fibers are found in the external coat. 

 The smaller vessels have only a connective-tissue coat lined with 

 endothelium. In the lymphatic vessels, as in the veins, are valves 

 opening toward the heart, but they are nearer together than are 

 those of the venous system. The origin of these vessels in the 

 tissues, as a rule, is by plexuses or by stomata, as in serous mem- 

 branes ; by blind extremities, as in the lacteals ; or by lacunar 

 interstices, as in some viscera and glands. They ultimately dis- 

 charge into the venous system on the right side through the 

 right lymphatic duct, and on the left side through the thoracic 

 duct. 



