S. S. mouse 



PARAFFIN SECTIONS 



137 



glass rod, or a piece of plastic, and bind 

 the tail to it with silk, being careful to 

 bind only loosely lest the imprint of the 

 silk show in the final section. Great care 

 must be tak(>n to keep the tail straight 

 along the glass; it is then attached in 

 front and in back to the body of the mouse 

 exactly parallel to the spine. If this is done 

 skilfully a median sagittal section will cut 

 through the central portion of the central 

 nervous system for its entire length and 

 will show a central section of the tail to 

 its very tip. It is just as well to kill the 

 whole htter and to prepare them in this 

 manner, since one or two specimens are 

 bound to get out of alignment in the 

 course of the subsequent operations. The 

 writer thought at one time that the 

 simplest method of maintaining the tail 

 straight would be to hang the mouse from 

 a loop of tape passed through its tail with 

 a weight attached to the nose, but the ob- 

 jection to this is that though the tail re- 

 mains dead straight, it does not remain 

 exactly parallel to the spinal cord. 



Before all this has been done, one should 

 have decided on the fixative to be em- 

 ployed and have made up a sufficient 

 quantity of it. Fixatives containing picric 

 acid should be avoided at all costs, since 

 the prolonged soaking in water Avhich must 

 inevitably accompanj^ decalcification will 

 cause the grossest sweUing and vacuolation 

 of picric-fixed materials. The writer's pref- 

 erence is for the dichromate-formalde- 

 hj'de-acetic mixtures, preference being 

 given to those which are based on the 

 original solution of Miiller and which thus 

 contain sodium sulfate. Numerous foi-- 

 mulas for these mixtures will be found in 

 Chapter 18 under the general hea(Ung 

 F 7000.1010, the writer has employed the 

 mixture Bohm and Opel 1907 with success 

 in a preparation of the present tj'pe. It 

 must not be imagined that this, or any 

 other fixative, will penetrate rapidly 

 enough to fix an entire mouse before con- 

 siderable autolysis has taken place in the 

 internal organs; it will be necessary to 

 make small openings in the sides if we are 

 to have a successful preparation. 



As soon, therefore, as the mouse has 

 been firmly fixed in the manner described, 

 a sharp scalpel should be taken and a 



series of slits made through the skin and 

 l)eritoneum along each side of the ab- 

 domen. These slits should not be more 

 than a milhmeter or so in extent, or there 

 may be a protrusion of the internal oi'gans 

 through them. Care must also be taken 

 not to cut the liver, or any major blood 

 vessel, or the entire abdominal cavity will 

 fill up with blood, and the appearance of 

 the finished preparation will be coni- 

 l)letely ruined. In addition to these shts 

 down the side, about one third of each 

 side of the head must be cut off with a fine 

 saw so as to expose the outer surface of 

 the brain. The use of bone forceps or wire 

 cutters will cause distortion; a fine jewel- 

 ers' saw is much better for the purpose. 

 The cutting of blood vessels in this case 

 does not make very much difference since 

 there are few cavities into which the blood 

 can flow. 



The mouse, having thus been bound to 

 its supports and a few small openings 

 made, is wrapped in a fold of cheesecloth 

 and suspended at about the center of at 

 least one liter of the selected fixative. The 

 jar containing the mouse and fixative 

 should then be placed in a dark place for 

 about two days. At the end of this time 

 the mouse is removed and the fixative is 

 replaced with new fixative. At this time 

 also extend the size of the openings which 

 have been made, since all of the blood 

 will now be coagulated and the internal 

 organs will be more or less firmly fixed in 

 place. In extending these openings, re- 

 member actually that no more than the 

 center half-millimeter of the mouse will 

 ultimately be required, and certainly all 

 of the limbs and considerable areas of both 

 flanks may be removed. Some experience 

 is necessary in deciding how^ much to 

 remove. This is another reason why the 

 entire litter of young mice should have 

 been sacrificed at the same time rather 

 than reliance placed on a single specimen. 

 The mouse should now be placed in fresh 

 fixative and left in the dark for a further 

 period of about a week, the jar being ex- 

 amined at intervals to make sure that the 

 fixative is not turning green. It will always 

 turn green-brown, but should it become of 

 a fairly dark-green color, it must immedi- 

 ately be replaced with new fixative. It is 



