XVIl] BLOOD VESSELS. CIRCULATION 135 



Tease it out in the preserving fluid ; it will be found to 

 consist almost entirely of elastic laminae. 



Observe the gradations from an almost homo- 

 geneous elastic perforated membrane to a meshwork 

 of elastic fibres. 



4. Remove a small portion of pia mater from the 

 brain of a recently killed animal, brush it well in 

 normal salt solution, and wash it in more salt solution, 

 make a moist film of it, fix with alcohol, stain with 

 haematoxylin (a trace of picric acid after-stain is 

 advantageous) and mount in balsam. Note 



The smallest arteries ; no distinct external coat is 

 seen; the middle coat consists of a single layer of 

 muscle cells wound transversely to the tube, the nuclei 

 are deeply stained, the outlines of the cells will be seen 

 on focussing; the inner coat is represented by the 

 elongated nuclei of the epithelioid lining. 



The capillaries showing as thin, nucleated, mem- 

 branous tubes. 



The small veins, of larger calibre than the small 

 arteries, with no muscular coat (this is special to the 

 central nervous system), and in general appearance 

 resembling the capillaries. 



5. A pithed frog is given you. Take one or two small pieces 

 of cotton-wool and plug the hole in the vertebral canal. Expose 

 the heart, cutting through the sternum in the middle line, and 

 pinning back the two parts. 



Cut away the exposed part of the pericardium, pass a thread 

 under the bulbus aortse, with fine-pointed scissors make a cut in 

 the bulbus near the ventricle. With a sponge, moistened with 

 normal salt solution, wipe away the blood which comes, stroking 

 the abdomen gently upwards to remove as much blood as 



