115 



LESSON XXIV. 



STRUCTURE OF THE HEART. 



1. IN a section through the wall of the auricle which has been stained with 

 magenta and mounted in glycerine, notice the relative thickness of the 

 epicardiurn, 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 logwood 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 columns carneae. 



3. 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. 



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

 as preparation 2. 



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

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

 been treated with nitrate of silver and subsequently exposed to the light and 

 hardened in alcohol. 



The muscular substance of the heart (myocardium] is composed of 

 transversely striated muscular fibres (fig. 139), which, differ from those 

 of voluntary muscle in the following particulars : their striations are 

 less distinct ; they have no sarcolemma ; they branch and unite with 

 neighbouring fibres, and their nuclei lie in the centre of the fibres. 

 Moreover, the fibres are composed of a series of short cylindrical cells 

 (fig. 140) joined together end to end, each corresponding to one of the 

 nuclei. The lines of junction of these cells may sometimes be seen in 

 longitudinal sections stained with hsematoxylin or magenta ; but they 

 come much more distinctly into view in sections of the fresh tissue 

 stained with nitrate of silver. 



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