STKUCTUKE OF THE HEART. 13<> 



LESSON XXVI. 



STRUCTURE OF THE HEART. 



1. IN a section through the wall of the auricle, stained with magenta and 

 mounted in dilute glycerine, or stained with hsematoxylin and mounted in 

 Canada balsam, notice the relative thickness of the epicardium, myocardium, 

 and endocardium. Observe the blood-vessels and nerve-fibres under the 

 epicardium, often embedded in fat ; here and there a ganglion may be seen 

 under this membrane. Notice also the elastic networks under both the 

 pericardium and endocardium. Make a general sketch from this section. 



2. Section through the wall of the ventricle, stained with hsematoxylin and 

 mounted in Canada balsam. The muscular fibres are variously cut. In 

 those cut longitudinally, notice the branching of the fibres and their union 

 into a network. Notice also that although the fibres are cross-striated this 

 is less distinct than in voluntary muscle, and the nuclei lie in the centre of 

 each fibre. Transverse markings may also be seen passing across the fibres 

 between the nuclei and indicating a division into cells. The endocardium is 

 very thin, especially over the columnae cariiese. 



3. If a portion of endocardium of the sheep's heart is spread out oil a slide 

 and examined in salt solution, a network of large beaded fibres may be seen 

 with a low power or even with a lens. These are the fibres of Purkinje, and 

 they will be seen to be formed of large, square-looking cells, usually contain- 

 ing two nuclei, and having striated muscular substance at their periphery. 



4. The lymphatics of the heart are easily injected with Berlin blue by 

 sticking the nozzle of the injecting syringe into the muscular substance, in 

 the interstices of which the lymphatics arise. These commencing lymphatics 

 lead to efferent vessels which pass to the base of the heart under the epi- 

 cardium. 



5. Section through one of the valves of the heart, stained and mounted as 

 in preparation 2. 1 



6. The epithelium which covers the epicardium, and that which lines the 

 endocardium, may be studied in preparations of the fresh organ which have 

 been well washed with distilled water ; then treated with nitrate of silver 

 and subsequently exposed to the light and hardened in alcohol. 



The muscular tissue of the heart (myocardium) forms the main 

 thickness of the ventricles and also of parts of the auricles. It is 

 composed of a network of fibres which are formed of uninucleated 



1 The appearances which are to be studied in sections 1, 2, and 5 can all be obtained in 

 one preparation, viz. a vertical section including a portion of auricle and ventricle and 

 a flap of the intervening auriculo-ventricular valve. 



