

HISTOLOGICAL METHODS. 153 



spinal cord, and nerve ; a dilute solution of chromic acid 

 with potassium bichromate is used for the brain, stomach, 

 and intestine. A solution of chromic acid in dilute alcohol 

 is used for the retina, cochlea, and skin (see pp. 4 and 5). 

 It is not well adapted for the liver, kidney, lymphatic gland, 

 crystalline lens, and vitreous humour. 



Chromic acid hardens the tissues in from two days to 

 two months. The tissue should be cut into small pieces 

 and placed in a small quantity of fluid for the first eighteen 

 hours. This should be changed at the end of this period, 

 and a large quantity of fluid substituted. The process of 

 hardening by means of chromic acid appears to be analogous 

 to the process of tanning. The tissue becomes tough, like 

 leather. The chromic acid is removed from the fluid, and 

 possibly forms some compound in the tissue, hence a large 

 quantity of the chromic solution is necessary. The solu- 

 tion employed is always dilute ; the same object cannot be 

 attained by using a strong solution of the acid, for in that 

 case an impervious crust forms around the outer part of the 

 tissue, while the centre remains soft and becomes rotten. 

 When the tissues are sufficiently hard, they are removed 

 from the acid and placed in methylated spirit, if it be 

 necessary to preserve them until sections are made. Very 

 delicate tissues that are apt to spoil in the chromic acid, 

 e.g. the embryo, should be placed in methylated spirit ere 

 they are sufficiently hard. The spirit completes the process 

 of induration. Spirit removes much of the yellow colour 

 given to the tissues by the chromic acid. 



Chromic acid is a very hygroscopic substance, and 

 therefore, as it is very difficult to keep dry, it is advisable 

 to keep it dissolved. A i per cent solution is most 

 convenient, and can be diluted at any time. 



275. Picric or Carbazotic Acid is a hardening agent 

 introduced by Ranvier. A saturated solution made with 

 cold water is employed. The pieces of tissue must be 

 small and the quantity of fluid large. 



It does not render the tissues so hard as chromic acid, 

 but at the same time its tendency to produce shrivelling is 

 less. It is recommended in 205 as a hardening agent 



