XIX.] THE HEART AND BLOOD-VESSELS. 225 



preparation is to make a transverse section across both ventricles of 

 the heart of a small mammal, e.g., guinea-pig. This is best done 

 in a heart stained in bulk. 



Sections may be cut by means of a freezing microtome, and then 

 stained with picro-carmine, and mounted either in Farrant's solution 

 or balsam. In both cases it is well to include a section of the peri- 

 cardium and also of the endocardium. Transverse sections of the 

 papillary muscles are very instructive. 



(a.) (L) Observe the branched and anastomosing fibres, but in 

 addition some of them will be cut obliquely, and others trans- 

 versely, with the nuclei stained (comp. p. 199). 



(b.) (H) The faint transverse striation, short branches, absence 

 of sarcolemma, the nucleus in the substance of the fibre, and the 

 indistinct cement substance (fig. 207). 



(c.) The fibrous character of the pericardium, which sends fine 

 septa between the bundles of muscular fibres. If the pericardium 

 be not included in the section, still connective tissue will be seen 

 between the bundles of fibres, especially in transverse sections of 

 these (fig. 207, B). 



2. Purkinje's Fibres (H) occur under the endocardium in the 

 heart of the sheep and some other animals. Open a ventricle of a 

 sheep's heart, observe the network 



of fine glistening lines ; strip off the 

 endocardium, snip out a little piece 

 of the heart-muscle, and place it for 

 thirty-six hours in dilute alcohol or 

 5 per cent, ammonium chromate for 

 two days. Tease a very small piece 

 in picro-carmine, and mount in 

 glycerine. 



(a.) Search for isolated polygonal 

 cells, each with usually two nuclei, 

 and the edges only of the cells 

 striated. These are heart-cells ap- FlG . 2oS ._^ injVs Fibres . Dilute 

 parently arrested in the process of alcohol, x 300. 



striation (fig. 208). 



3. Endocardium (H). Harden a part of the ventricle of the 

 human heart in alcohol or potassic bichromate. Make sections tc 

 include the endocardium. 



(a.) Observe oval nuclei on the surface, the nuclei of the endo- 

 thelial cells. Under this a superficial layer of fibrous tissue (fig. 

 209, a), with a few smooth muscle-cells (?/), and underneath this 

 fibrous tissue the basis of the membrane (tc). 



(b.) Outside this is the myocardium (me). 



4. T.S. Heart-Valve (H). Harden a cusp of a human tricuspid 



21 P 



