402 HISTOLOGY. 



rated solution of potassium chlorate in nitric acid. (About 5 gr. of potas- 

 sium chlorate should be added to 20 cc. of nitric acid.) The muscle fibers 

 should be separable in from i to 6 hours. They should be washed in 

 distilled water for an hour or a few days so as to remove the acid, and then 

 may be examined in water or in glycerine. 



Other macerating fluids are 10 to 20 per cent, nitric acid, diluted either 

 with water or with salt solution; -g-g-^ to -i of i per cent, of chromic acid; 

 and water, by which the pulpy portion of organs may be removed from the 

 connective tissue framework. Complex but valuable methods for demon- 

 strating the connective and reticular networks have been described by 

 Mall and Flint. They involve digestion of the tissues with pancreatic 

 extract. 



SECTIONING FRESH MATERIAL. 



Since the cutting of freehand sections of fresh tissue held between 

 pieces of pith is no longer practised, the most rapid method for obtaining 

 sections is by means of the freezing microtome. Small blocks of fresh tissue 

 not over 5 mm. thick are moistened with water and placed upon the carrier 

 of the microtome, where they are frozen by a jet of carbon dioxide pro- 

 ceeding from a cylinder of the liquefied gas. Sections 10-15 f l thick may 

 be chiselled from the frozen tissue and placed in a dish of water, in which 

 they unroll. Then they are floated upon a slide and may be stained by 

 ordinary methods. Frozen sections may be made from tissue hardened in 

 formaline as well as from fresh material. In some cases this method is of 

 special value in studying normal tissue; for rapid diagnosis of pathological 

 conditions it is indispensable. 



Descriptions of the freezing and other microtomes with full directions 

 for their use will be found in Mallory and Wright's "Pathological Tech- 

 nique." The use of the instruments, however, is seldom learned except by 

 personal demonstration in the laboratory. 



FIXATION. 



The fixation of tissues is the process by which post mortem changes are 

 prevented, mitosis, for example, being checked at once and the mitotic 

 figure permanently preserved. The hardening of the tissue is completed 

 subsequently by immersion in alcohol. Small blocks of the desired tissue 

 (about i c.c. in volume and preferably less than i cm. thick) should be 

 dropped without handling into a considerable quantity of the fixing fluid. 

 Contact between the fingers and the peritonaeum is sufficient to destroy 

 the thin mesothelium. It is often advisable to place a piece of absorbent 

 cotton beneath the tissue so that the fixing fluid may have access to its 



