120 PRACTICAL HISTOLOGY [XV 



tissue round the auricles, and two-thirds of the ventricle. Move 

 about in the water, and place in 25 p.c. formic acid in the dark 

 till next day. 



Then wash in water, and under a dissecting lens cut through 

 the projecting wall of one auricle, note the position inside of the 

 thin membrane, the septum auricularum, and cut away from it 

 the auricles and the rest of the ventricle. Mount it in formic 

 glycerine. Note (a) the two main strands of nerve fibres running 

 through it ; the medullated fibres in these will be stained more 

 deeply than the non-medullated fibres. (&) The septum itself 

 formed of a plexus of cardiac muscle cells in a connective 

 tissue membrane, (c) The numerous nerve cells attached to 

 the nerve-strands. The nerve cells are generally pear-shaped. 

 The spiral process may be seen coiling round the straight pro- 

 cess, but it is not usually distinct. 



(The heart may of course be injected from the inferior vena 

 cava ; the method described above is recommended because it is 

 perhaps easier to insert a cannula into the bulbus than into the 

 vena cava.) 



The heart may be injected with alcohol ; in this case inject weak 

 before strong alcohol, and colour the strong alcohol with a little 

 eosin, so that the septum may be more easily seen. Pass scissors 

 down the bulbus into the ventricle arid cut ; cut off three-fourths of 

 the ventricle, pass scissors from the ventricle into one auricle and cut. 

 The septum can then be readily separated under a dissecting leiis, and 

 stained with any reagent desired. 



9. Nerve cells of Auerbach's plexus. Cut through and reflect 

 the abdominal walls of a recently killed mouse or rat. Cut out 

 about two inches of the small intestine, tie a cannula in one end, 

 and using a syringe wash out the intestine with normal salt 

 solution. Tie the free end. With a pipette fill the cannula 

 with 1 p.c. gold chloride, distend the intestine with this, tie just 

 below the cannula, whilst the intestine is still distended. Place 

 it in a watch-glass in gold chloride solution for ^ to 1 hour. Re- 

 move to water ; cut open lengthways, and wash well. Place in 

 25 p.c. formic acid in the dark till next day. 



