BLOOD. 29 



the disks run from l-2500th to l-5000th of an inch ; but 

 that the great majority are about l-3300th in diameter. 

 On these slides are samples of other kinds. This is the 

 blood of a Fish the common Blenny or Shanny (Blen- 

 nius pholis). Here we see at once the oval form of the 

 disks ; their average is 1 -2800th by 1 -3300th of an inch. 

 Here is the blood of a Frog (Rana temporaria) ; these are 

 more than twice the size of the fish's ; for they average 

 l-1250th by l-1800th of an inch. And, finally, I can 

 show you a drop of blood from this Smooth-newt (Lisso- 

 triton punctatus). The large size of the disks is now 

 plainly seen, and so indeed is the elegance of their form : 

 in this case, as in the last, we see in each disk a distinct 

 roundish nucleus. These run from l-703th to l-950th in 

 length by l-1100th to l-1600th in breadth; but the 

 average are about l-800th by l-1300th of an inch. 



It may interest you to see these blood disks in their 

 proper situation, and to observe the motion which they 

 possess during the life of their owners. It is, indeed, one 

 of the most instructive modes of using this wonder-work- 

 ing instrument to look through it at living structures, and 

 watch the different processes of life as they are carried on 

 under our eyes. Nor is this at all difficult to accomplish ; 

 for a large number of animals are so small that we can 

 easily put them upon the stage of the microscope ; and 

 are withal so transparent that their coverings and various 

 tissues offer little or no impediment to our discerning the 

 forms and movements of the contained viscera. And in 

 cases where the entire animal is too large to be viewed 

 under the microscope as a whole, it sometimes happens 

 that, by a little contrivance, we can so secure the creature 

 as to look without interruption on certain parts of the body 

 which afford the requisite minuteness and transparency. 



I have here a living Frog. You perceive that the web 

 which connects the toes is exceedingly thin and trans- 

 lucent, yet arteries and veins meander through its delicate 



