30 EVENINGS AT THE MICEOSCOPB. 



the Elephant, and of the Kangaroo. Most other quad- 

 rupeds have them smaller than in Man ; the smallest 

 of all being found in the ruminating animals. The 

 little Musk-deer of Java has disks not more than one- 

 fourth as large as the human, but these are remarkably- 

 minute ; no other known animal approaches it in this 

 respect : those of the Ox are about three-fourths, and 

 those of the Sheep little more than half the human 

 average. 



Tables have been made out showing the compara- 

 tive size of these corpuscles in various animals, and 

 such tables are very useful ; but we mnst bear in mind 

 that the average dimensions only are to be looked for ; 

 since in any given quantity of blood, under examina- 

 tion, we shall not fail to see that some disks exceed, 

 while others come short of, the dimensions of the ma- 

 jority. 



Generally speaking, the blood-disks iu Birds and in 

 Fishes are aboiit equal in size : their form is, however, 

 tliat of a more elongated ellipse in Birds than in Fishes. 

 They may be set down as averaging in breadth the 

 diameter of the human disks, while their length is 

 about half as much again, or a little more, in most 

 Birds. 



It is in Eeptiles that we meet with the largest disks, 

 and especially in those naked-skinned species, the Frogs 

 and ISTewts. A large species inhabiting the American 

 lakes — Siren lacertina — has disks of the extraordinary 

 size of l-400th of an inch long by l-800th broad, or 

 about eight times as large as those of Man. Our com- 

 mon Newts afford us the largest examples among Brit- 

 ish animals, but they do not reach above half the size 

 just mentioned. 



