BLOOD CRYSTALS. 551 



determine their shape; they rapidly move lengthways, the 

 entire field of vision being gradually covered by a dense 

 network of acicular crystals, crossing one another in every 

 direction, with other crystals presenting the form of rhombic 

 plates. 



Dr. Garrod discovered, that by a slow evaporation of 

 portions of the serum of blood taken from patients labour- 

 ing under gout, he could obtain strings of crystals of uric 

 acid. His mode of procedure is to pour a little serum 

 into a watch-glass, and add a few drops of acetic acid ; 

 in this mixture place a few very fine filaments of silk or 

 tow, and stand it by for twenty-four hours under a glass- 

 shade. Upon removing the glass and submitting the fila- 

 ments to microscopical examination, they are seen to be 

 studded with minute crystals of uric acid. 



No. 1, fig. 285, the foot of the Frog is stretched out, to 

 show the distribution of the blood-vessels in the web : the 

 two sets of vessels the arteries and veins are very readily 

 made out when kept steadily on the stage of the microscope; 

 the rhythm and valvular action of the latter may be 

 observed, although they are much better seen in the ear 

 or wing of the Long-eared 

 Bat, as first pointed out by 

 Professor Wharton Jones. 



The circulation in the foot 

 of the Frog and the tail of 

 the Newt is, for the most 

 part, the capillary circula- 

 tion. The ramifications of 

 the minute arteries form a 



COntinUOUS network, from Fig. 2B6.IIead of lon.g-eared Bat. 

 , . , ,, ,, , , p (Piecotus Aurilus.) 



which the small branches 01 



the veins take their rise. The point at which the arteries 

 terminate and the minute veins commence, cannot be 

 exactly defined ; the transition is gradual ; but the inter- 

 mediate network is so far peculiar, that the small vessels 

 which compose it maintain nearly the same size through- 

 out ; they do not diminish in diameter in one direction, 

 like arteries and veins ; hence the term capillary, from 

 capillus, a hair. (Fig. 287.) The size of the capillaries 

 is proportioned in all animals to that of the blood cor- 



