THE HEARTS BEAT 153 



individuals of this group. It is an oblong sac with branch- 

 ing vessels at either end. It beats for some thirty or forty 

 strokes so as to drive the blood forwards ; it then pauses, 

 and the onlooker is astounded to see the wave of move- 

 ment changed, and the heart steadily beating the same 

 number of strokes in the reversed direction. What were 

 arteries become veins, and the veins become arteries. Then 

 again there is a pause which seems like a moment of 

 hesitation and doubt and the original direction of move- 

 ment is resumed ; then again there is a pause and a 

 reversal, and so on, with absolute regularity. It is still 

 a matter for investigation as to why and how this alto- 

 gether exceptional alternating reversal of the heart's action 

 is brought about. 



It is a curious fact in illustration of the essential 

 character of the heart and its beat that "hearts " are pro- 

 duced in some animals by dilatation of the lymph-vessels 

 a system of delicate vessels, difficult to see, which take 

 up the colourless fluid which the blood-vessels exude into 

 the tissues and return it to the heart. The eel has a pair 

 of these " lymph-hearts " in its tail, and the common frog 

 has a pair near the shoulder-blades and another pair at 

 the hips. These sacs have muscular walls, and pulsate 

 rhythmically like the blood-heart, driving on the lymph 

 fluid through the lymph vessels to join the blood-stream. 



The simplest thing in the animal world which can 

 claim the name of a heart or, at any rate, be compared 

 with that organ is found in those microscopic animal- 

 cules which consist of only a single " cell " or corpuscle 

 of living protoplasm. These animalcules may be com- 

 pared to a single brick or unit of structure, whereas all 

 other animals consist of thousands, or even millions, of 

 such corpuscles or units aggregated and fitted together 

 as are the bricks and planks of a house. In most of 

 these uni-cellular animalcules you may observe with a 



