BLOOD CRYSTALS. 363 



nous crystals, — which, though formed of animal matter, and sometimes, 

 in all probability, during life, have forms as regular as any inorganic 

 crystals. 



Various authors, Sir E. Home, Scherer, and others, described reddish 

 crystals in blood which had been effused into tissues or organs ; but 

 Virchow was the first who paid particular attention to their actual nature, 

 and proved them to differ from saline or earthy crystals. If we add water 

 to a drop of blood spread out under the object-glass of the microscope, 

 as the drop is beginning to dry up the edges of the heaps of blood cor- 

 puscles are seen to undergo a sudden change : a few corpuscles dis- 

 appear, others have dark thick edges, become angular and elongated, 

 and are extended into small well-defined rodlets. In this manner an 

 enormous quantity of crystals are formed, which are too small to en- 

 able us to 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 labouring under gout, he 

 could obtain strings of crystals of uric acid : this may prove of great 

 value as a diagnostic sign of this disease. His mode of proceeding is 

 to pour a little serum into a watch-glass, and add a few drops of acetic 

 acid ; place in this mixture a few very fine filaments of silk or tow, 

 and stand it by for twenty-four hours under a glass-shade. Upon 

 removing and submitting the filaments to microscopical examination, 

 they will be seen to be studded with minute crystals of uric acid. 



No. 1, fig. 172, the foot of the frog 

 is stretched out, to show the distribu- 

 tion of the blood-vessels in the web : the 

 two sets of vessels — the arteries and 

 veins — are very readily made out when 

 kept steadUy on the stage of the micro- 

 scope ; the rhythm and valvular action 

 of the latter may be observed, although 

 they are much better seen in the ear ^ead of Long-trll'Bat. PUcotu, 

 or wing of the long-eared bat, as first AurUus. 



pointed out by Wharton Jones. 



The circulation in the foot of the frog and the tail of the newt is, 

 for the most part, the capillary circulation. The ramifications of the 

 minute arteries form a continuous network, from which the small 

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



