THE MICROSCOPE IN ANIMAL HISTOLOGY. 187 



foot. The circulation may be readily observed by ether- 

 izing a frog, and expanding its foot by means of pins or 

 thread, upon the stage of the microscope (Plate XVIII, 

 Fig. 137). The circulation may also be seen in the lung, 

 mesentery, or extended tongue, of the frog. 



The red corpuscles of blood are flattened disks, which 

 are circular in Mammals, except the camel and lama, 

 which have elliptic disks. In birds, amphibia, and most 

 fishes, the disks are elliptic. In a few fishes (the cyclos- 

 tomata) they are circular. Their color depends on haemo- 

 globulin, which plays an important part in the exchange 

 of respiratory gases. In man the disks are usually double- 

 concave, with rounded edges. Out of the body they have 

 a tendency to adhere, or run together, in chains, like rolls 

 of coin (Plate XVIII, Fig. 138). In the elliptic disks of 

 birds, etc , there is a distinct nucleus. The size of the 

 disks varies. In man they are from 0.0045 to 0.0097 mil- 

 limetre. The smallest disks are in the Moschus Javanicus, 

 and the largest in Siren lacertina. In the latter they are 

 from ^g to g 1 ^ millimetre. 



It is estimated that in a cubic millimetre (about J-ih 

 of an inch) of human blood there are 5,000,000 red cor- 

 puscles, having a surface of 643 millimetres. 



After a variable time from their removal from the ves- 

 sels they suffer contraction, and assume a stellate, or 

 mulberry form (Plate XVIII, Fig. 139). This occurs 

 more rapidly in feverish states of the system. On the 

 warm stage they suffer still greater alterations. Inden- 

 tations appear, which cause bead-like projections, some 

 of which become fragments, having molecular motion 

 (Plate XVIII, Fig. 139^. The substance of red corpuscles 

 is elastic and extensible, and may be seen in the vessels to 

 elongate and curve so as to adapt themselves to the calibre 

 of the vessels. 



Electric discharges through the red corpuscles produce 

 various changes of form. Alkalies dissolve, and acids 



