18 CYTOLOGICAL TECHNIQUE 



it is usual in practice to mix two or more of them together. 

 Fixative mixtures will be considered in chapter 5 (p. 59). Their 

 effects on tissues can only be understood when those of their 

 components are known. Further, the proportions of the primary 

 fixatives in the mixtures have been very arbitrarily chosen. For 

 these reasons, any scientific account of fixation must start with 

 the primary fixatives and be concerned mainly with these. 



Small pieces of tissue are best fixed by direct immersion, since 

 this brings the fixative most rapidly into contact with the cells 

 throughout the piece. When it is necessary to fix a piece of tissue 

 a centimetre or more thick, it is best to inject the fixative through 

 a blood-vessel in order to send it quickly to all depths. This 

 method of 'perfusion' has the disadvantage that the total amount 

 of fixative that can be contained in the blood-vessels is usually 

 small, and nothing outside the vessels is fixed until the fixative 

 substance has passed through their walls into the intercellular 

 spaces and thus reached the cells. 



The purpose of fixation is usually to stabilize the tissues so that 

 they retain as nearly as possible the form they had in life, but 

 clearly it is not the purpose to leave their chemical composition 

 unchanged. This would indeed be the negation of fixation, for 

 the cells would be still alive or like recently dead ones. The 

 purpose is to change the chemical composition in such a way as 

 to confer structural stability. This fact by no means makes 

 histochemical studies impossible, for certain tissue-constituents 

 may be left unaltered, and others only altered in part. 



The structural formula for amino-acids in general is con- 

 veniently written in the way shown here, because such formulae 

 may easily be joined together to make a protein chain. The letter 

 R in the formula represents any of the radicles that distinguish 

 the various amino-acids from one another. In forming a protein, 

 the amino-acids condense together with elimination of one 



( II H 

 molecule of water at each peptide link \-C — N-> 



In the formula representing part of a protein chain, certain 



conventions have been adopted. The radicles characteristic of 



