32 



EVENINGS AT THE MICKOSCOPE. 



The larger ones do indeed ; and their course is from the 

 extremity of the toes towards the body : these are the 

 veins ; but the smaller streamlets flow in any direction, 

 and frequently send out side -branches, which presently 

 return into the stream from which they issued, or unite 

 with others in a very irregular net- work. These are the 



capillaries which feed 

 the veins, and which 

 are themselves fed by 

 the arteries, whose 

 course is in the oppo- 

 site direction, viz., 

 from the body. These, 

 however, are with dif- 

 ficulty seen : they are 

 more deeply seated in 

 the tissues, and are 

 less spread over the 

 webs, being generally 

 placed along the bor- 

 ders of the toes ; they 

 are, moreover, fewer 

 and smaller than the 

 veins ; but the blood 

 in them usually flows with more impetuous rapidity. 



The variations in the impetus of the current which we 

 observe in the same vessel are probably owing to the men- 

 tal emotions of the animal ; alarm at its unusual position, 

 and at the confinement which it feels when it endeavours 

 to move, may suspend the action of the heart, and thus 

 cause an interruption in the flow ; or analogous emotions 

 may quicken the pulse. We will, however, now release 

 our little prisoner, who, though glad to be at liberty, is, as 

 you see, none the worse for his temporary imprisonment. 



Let us now look at the circulation of the blood in one of 

 the Invertebrate Animals. In this thin glass cell of sea- 



CIHCULATION IN FROG'S FOOT. 



