180 APPLIED PHYSIOLOGY 



but not enough to force it along the lymphatics. Its flow 

 is aided by the pressure of muscles upon the spaces and 

 the tubes. Its current is slow and unsteady. It is finally 

 poured through the thoracic duct into a vein at the root of 

 the neck, where it mingles with the blood. About two 

 quarts of lymph pass through the thoracic duct daily. If 

 a hollow needle is thrust into the skin, and through it 

 water containing medicine is forced, the medicated water 

 spreads through the lymph spaces between the living 

 cells. Some is taken up by the capillaries, and some passes 

 into the circulation by means of the lymph, and produces 

 the same effect as though it entered the blood through the 

 stomach. 



Sometimes the lymph cannot be removed by the lym- 

 phatics so fast as it is poured out by the capillaries. It 

 then distends the lymph spaces, producing uniform 

 swellings called dropsy. Dropsy can. be recognized by a 

 small pit remaining when the finger is pressed into the 

 skin. 



314. The circulation in lower animals. Land animals 

 and birds possess a heart and blood tubes like man's, and 

 their circulation follows the same order. The heart of rep- 

 tiles and toads consists of two auricles and one ventricle, 

 and the ventricle always contains both arterial and venous 

 blood. 



Fishes possess only one auricle and one ventricle. The 

 ventricle forces the blood through two sets of capillaries, 

 and the circulation is made correspondingly sluggish. 



Insects possess a row of eight or nine sacks connected 

 by a tube, with valves opening toward the head. The 

 contraction of the sacks forces the blood toward the head, 

 where it escapes into the lymph spaces between the cells. 

 There are no arteries or veins, and so the blood is slowly 



