HARVEY AND THE CIRCULATION. 245 



tivity are cut off or exhausted. The moving forces of 

 a living organism are largely dependent upon one 

 another. In the lower animals a tentacle may he 

 lopped off without disturbing the creature much, and 

 a new part may he reproduced. A hydra may be 

 divided in the middle from rays to foot-stalk, and the 

 halves will soon become converted into two individuals^ 

 each having a set of independent nerve centers which 

 can not be discommoded by decapitation or dismem- 

 berment. In the lowest vertebrates, such as salaman- 

 ders, the body may be cut into a dozen pieces, and for 

 hours each segment will respond to irritation as if the 

 entire living body were present. The circulation in 

 such creatures is kept up to a certain extent in the 

 various sections. No heart is needed, but the walls of 

 the vessels, under the stimulation of isolated and inde- 

 pendent nerve ganglia, will keep the blood moving, 

 rather sluggishly, through the capillaries. 



In the forming embryo chicks pulsating tube or 

 sac is observed before a globule or red blood can be 

 seen. A few nerve cells in the walls of the rudiment- 

 ary heart are the stimulus of the pulsatory motion. 

 Furthermore, oxygen is essential to activity in the 

 nerve battery, for as soon as an incubated egg is var- 

 nished to exclude air from the yelk within, the cardiac 

 pulsation ceases, and the embryo dies. This shows 

 that aeration is an essential feature of life, or vital 

 activity. 



In regard to the lacteals and lymphatics, it may 

 be said that they belong to the circulatory system, and 

 pour their contents into the veins near the heart. 

 The lacteals take up nutriment from the mucous sur- 

 faces of the intestinal track, and the lymphatics ab- 

 sorb such material in various parts of the body as is 



