BLOOD 41 



The blood-disks of Man nearly agree in size with those- 

 of the Monkey tribe, of the Seals and Whales, of the Ele- 

 phant, and of the Kangaroo. Most other quadrupeds 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 comparative 

 size of these corpuscles in various animals, and such tables 

 are very useful; but we must bear in mind that the aver- 

 age dimensions only are to be looked for; since in any 

 given quantity of blood, under examination, we shall not 

 fail to see that some disks exceed, while others come short 

 of, the dimensions of the majority. 



Generally speaking, the blood-disks in Birds and in 

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

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

 They may be set down as averaging in breadth the diam- 

 eter 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 Newts. A large species inhabiting the American lakes 

 Siren lacertina has disks of the extraordinary size of 

 l-400th of an inch long by 1 -800th broad, or about eight 

 times as large as those of Man. Our common Newts afford 

 us the largest examples among British animals, but they 

 do not reach above half the size just mentioned. 



Taking this drop of blood from my finger as a standard 



