CHAPTER IV 

 THE CARPS— Continued 



As sufficient information cannot be obtained of the various lobes of 

 Engraulicypris by the naked eye, it becomes necessary to employ 

 another method of observation ; this is the method of examining 

 the internal structiu-e of an organ by cutting serial sections. To 

 those Avho are unfamihar with this method, the following sketch 

 of the process may be useful. The tissue to be examined is first 

 " fixed," that is to say, put in a solution, which prevents chstortion 

 or destruction of the cellular elements, which might occur in the 

 later stages of the process. After being fixed, the tissue is 

 " hardened " by various reagents ; in the case of brain tissue, the 

 cheesy consistence is changed to that of soft leather. The specimen 

 is now embedded in paraffin of low melting point and allowed to 

 stand in an incubator until the paraffin has thoroughly permeated 

 the tissue. It is then taken out and allowed to cool. The sohd 

 paraffin block is then put in a proper position on a microtome and 

 very thin shces are cut in series, which come away from the micro- 

 tome in ribbons, lengths of which are placed on shdes, and numbered.. 

 The paraffin is then dissolved out, and the sections stained in the^ 

 appropriate manner to show up details. These are examined 

 seriatim and successive drawings made under the microscope. 



Suppose the first drawing was made of a transverse section of 

 the hinder end of the medulla, each section, as you pass forwards, 

 is studied and, when a modification of the picture occurs, a fresh 

 drawing is made and so on, until you have examined and made 

 pictorial notes of the whole of the medulla. In this way it is possible 

 to make a mental picture and build up the structure of the various 

 lobes and plot them out. Also the intimate details of the tissue are 

 made clear, and the different types of cells recognised, and the course 

 of the connecting strands of nerve fibres foUowed. The latter often 

 form definite tracts to which terms are applied, signifying their 

 supposed functional significance. 



To help the reader to follow the interpretation of the series of 

 transverse sections of the medulla oblongata and cerebellum of 

 the roach, we give a half-a-dozen diagrammatic drawings of this 



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