XIX.] 



THE HEART AND BLOOD-VESSELS. 223 



acts on the nerves, and on the latter being exposed to the air they become 

 blue. It may also be applied to fresh tissues. 



Inject some of the following solution into the blood-vessels : 



Methylene-blue I gram. 



Normal saline .... 300 cc. 



Or introduce a 3 per cent, solution, or even the solid substance, into the 

 'lymph-sac of a frog. After an hour or two, expose a muscle to the air, 

 or use the cornea, or any other tissue with nerves in it, and on examining 

 it under the microscope the nerves will be found stained blue. To preserve 

 such specimens, mount them in a solution of picrate of ammonia and glycerine 

 (p. 192). Fresh tissues, e.g., cornea or a thin muscle, may be immersed in a 

 weak solution of methylene-bliie with the sjime result. 1 



The methylene-blue method may boused for the study of nerve terminations 

 in any organ, e.g., the sense organs, and in arteries one can see most beauti- 

 fully the plexus of non-medullated fibres in the muscular coat. 



19. Nerve-Cells of Crayfish. Select a small individual and inject into its 

 abdominal cavity I to 2 cc. of a 0.2 per cent, solution of methylene-blue. 

 After 8 or 10 hours, remove the chitinous covering over the ganglia ted nerve- 

 cord and expose the latter in a vessel, which admits air and yet prevents 

 evaporation. In 24 hours or so excise a ganglion and observe it in a drop of 

 glycerine tinged with picrate of ammonia (Ketzius). 



LESSON XIX. 

 THE HEART AND BLOOD-VESSELS. 



Heart. The wall of the heart consists of (i.) Pericardium; 

 (2.) Myocardium ; (3.) Endocardium. 



The pericardium covering the heart is a serous membrane 

 composed of fibrous tissue, with numerous elastic fibres, and covered 

 on its free surface by serous endothelium. It is sometimes called 

 epicardium. The fibrous tissue is continuous with that which 

 invests the bundles of muscles of the myocardium itself. Under- 

 neath the epicardium are the blood-vessels, nerves (ganglia), and 

 the lymphatics. 



The myocardium is composed of striated muscular fibres, whose 

 characters have been described already (Lesson XVI. 12). The 

 iibres are arranged in bundles separated from each other by a 

 greater or less amount of connective tissue, in which run the blood- 

 vessels and nerves. 



The endocardium in structure resembles the pericardium, but 

 it is thinner. It consists of a fibrous basis, with elastic fibres 

 covered by a single layer of endothelium. It contains a few smooth 

 muscular iibres. 



An artery consists of three coats : 



(i.) Tunica intima, or inner coat, composed of a single layer 



1 S. Mayer, Zcitsch. f. wiss. Mikrox., vi. 422, 1889, gives numerous references. 



