HANDLING THE MATERIAL 2()9 



should be well filled with the fixative so that there will be many times 

 the volume of the tissue to be fixed. The small amount of water 

 from the tissues should not dilute to any a])])reciable dej^ree the 

 concentration of the fixing fiuid. 



HANDLING THE MATERIAL. 



Dissection. — Pieces of tissue should be small. This is important 

 for good fixation. Generally pieces, 0.5 cm. to 1 cm. in thickness, 

 can be used. The length and breadth of the piece should likewise 

 be kept to the smallest practical size. The pieces should be handled 

 as little as possible with dissecting instruments and care should be 

 taken not to compress them during dissection. The operator should 

 keep in mind that he is later going to make sections of these pieces, 

 and should remove material from the desired portion of the organ 

 in the proper manner for longitudinal, cross-, or tangential sections. 

 The selected pieces can be transferred from the animal to the bottles 

 of fixing fluid with a spatula, or lifterl lightly with forceps, without 

 permitting the instriuuents to enter the fluid. If dissection is pro- 

 longed, it is advisable to keep the organs of the freshly killed animal 

 moistened with the proper physiological saline solution (0.85 ])er 

 cent sodium chloride for mammals; 0.7 per cent for amphibians). 



Foreign Matter. — In fixing parts of the alimentary canal, it is 

 necessary to remove the contents of the lumen. This may be done 

 by washing out with ])hysiological saline before cutting out pieces 

 for the fixing fluid. Or fixing fluid may be forced under gentle 

 pressure into the limien of the canal with a pipette. If the bladder 

 is dilated with urine, the neck of the bladder can be ligated, and then 

 the entire bladder removed and placed in a beaker of fixing fluid for 

 a few minutes to stiffen the wall in the extended position. The 

 bladder then can be opened, emptied, and so cut that fixative can 

 freely come in contact with the internal surface. The stomach can 

 be handled in the same fashion. Or a small strip of stomach wall 

 can be placed on a strip of stitt' paper and thus inunersed in the 

 fixative for a few minutes, and then, when stiffened in the extended 

 condition, it can be removed from the paper and entirely immersed 

 in fixative. Pieces of artery or ner\'e can likewise be placed extended 

 on a paper strip, and this placed in fixative until the pieces are stiff 

 enough to be removed from the paper for further fixation. 



Heat. — Fixatives are ordinarily used at room temperatures, but 

 the effect is hastened by increasing the temperature. Heat itself 



