CHAPTER XXIV. 



STRUCTURAL CHANGES DUE TO AND FOLLOWING 



DAMAGE. 



I. NECROSIS. 



THE term necrosis designates a local death of tissue during the 

 life of the individual. 



In our study of the normal tissues under the microscope we are 

 obliged to use methods of preparation which, in nearly all cases, 

 kill the tissues before they come under observation. When we 

 examine them with a view to determining their structure, they are 

 nearly always necrotic, if we may use that term in this connection. 

 Our standards of the normal appearances are, therefore, largely 

 based upon what we learn from recently killed tissues. 



In some instances it is possible, however, to examine even highly 

 developed tissues while still living. If, for example, the super- 

 ficial layer of a frog's cornea be stripped off and mounted in a 

 drop of serum, the cells composing it may be readily seen under the 

 microscope. While such a preparation is quite recent it is difficult 

 to distinguish clearly the nuclei within the cells, their refractive 

 indices being nearly the same as that of the surrounding cyto- 

 plasm ; but in a short time the nuclei suddenly become very distinct, 

 as though they had undergone a sort of crystallization. This is 

 probably an indication of the death of the nuclei, the substances 

 composing them having suffered a coagulation which increases their 

 powers of refracting light and, in consequence, the distinctness with 

 which they are seen. This conclusion is strengthened by the fact 

 that the change may be hastened by the application of reagents, such 

 as acetic acid. 



The modern methods of preparation used in histological studies 

 aim at bringing about a sudden death of the cells and such a coag- 

 ulation of the tissue-elements as shall prevent further changes of 

 structure before the tissues can be studied. For, if the tissues are 

 allowed to die spontaneously, their elements suffer changes that 

 greatly alter their appearance. When they die and remain within 



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