THE HEART. 193 



CHAPTER XI. 



THE HEART. 



Preparation 1. The cardiac pericardium. 



The pericardium which covers the surface of the 

 heart is prepared by the same methods as other serous 

 membranes. Of these the only one which need here 

 be described is that by nitrate of silver. This is as 

 follows : In an animal which has just been killed the 

 thorax is freely opened, and the pericardium having 

 been torn open, the base of the heart is secured by a 

 tape ligature, the great vessels being then cut 

 beyond the ligature, and the organ removed without 

 allowing its surface to be smeared with blood. A 

 part of the surface is now brushed firmly with a soft 

 camel-hair brush moistened with distilled water, 

 with the object of removing the superficial layer of 

 epithelioid cells. The heart is next dipped for a 

 moment in distilled water, and then nitrate of silver 

 solution is poured over the whole surface, and al- 

 lowed to be on it for three minutes, after which the 

 organ is rinsed again in distilled water, and finally 

 placed in spirit, in the sunlight. AVhen sufficiently 

 browned it is removed from the window, and left 

 for some hours until the surface is hardened by the 

 alcohol. Surface sections are then made of both un- 

 brushed and brushed parts, and after soaking in 

 water for a minute or two are mounted separately 

 in glycerine. The unbrushed specimens will show 

 the epithelioid layer ; the brushed ones should 

 exhibit the subjacent connective tissue, with its 

 cell-spaces, lymphatics, bloodvessels, and nerves. 



Preparation 2. The muscular substance of 

 the heart is studied in teased preparations and in 

 17 



