SECTIONING OF TISSUES. 387 



Many other fluids may be used for special preparations 

 (e. g., nitric acid (5 per cent.) for the isolation of muscle 

 elements, sodium bicarbonate for the demonstration of base- 

 ment membranes, pancreatin in the study of connective tissue, 

 and the various destructive methods in the isolation of the 

 frameworks of organs). 



(b) Sectioning of Tissues. 



Another method of demonstrating the finer structure of 

 tissues is by means of sections. These are thin slices of the 

 tissue (1-50 11 thick), which are cut with a sharp knife either 

 free hand or with the help of an apparatus known as the micro- 

 tome. Free-hand sections have the advantage of being obtained 

 more quickly and of not requiring such elaborate preparation. 

 On the other hand, they cannot be cut sufficiently thin for all 

 purposes; they are not of uniform thickness throughout; and 

 it is impossible to cut two successive sections exactly the same 

 thickness. The microtome is an instrument which is so arranged 

 that the object to be cut is fixed on a stand which may be 

 raised one or more micromillimetres at a time. The knife is 

 fastened to a sliding stand, so that a section is cut each time it 

 is drawn back, and the object stand raised. Other instruments 

 have the knife stationary, and the object stand connected with 

 a wheel which on each revolution advances the object toward 

 the knife a certain number of micromillimetres. There is an 

 apparatus attached to the instrument by which the thickness 

 of the sections may be regulated. By means of this the mi- 

 crometer screw which raises the object may be set so that the 

 sections are all cut 5, 10, or 15, etc., micromillimetres thick. 



In order to obtain sections of this kind, the tissues should 

 be prepared by hardening and fixing them, so that they are 

 firm. This may be done by freezing. Various freezing mixt- 

 ures have been employed, but the best results can be obtained with 

 the carbon dioxide apparatus. Several instruments have been 

 devised for the use of this, but by far the best is that invented 

 by Bardeen. Frozen sections cannot be cut with regularity 

 thinner than 15 ^. They are mainly useful where it is neces- 



