34 THE MYSTERY OF LIFE. 



let us take a small portion of its tissue and 

 examine it. The thin muscular wall of the 

 auricle of the heart of a young frog, or, still 

 better, of the tree frog (Hyla), is well adapted for 

 this purpose, as it is so very thin that no cutting 

 or dissection is required in order to make it 

 thin enough for examination even with the 

 highest magnifying powers. It is so transparent 

 that every texture entering into its structure 

 can be seen in a properly prepared specimen. 



The drawing in Plate I will give some idea 

 of what can be made out. The network of 

 branching fibres seen in every part of the speci- 

 men, and of different degrees of fineness, is mus- 

 cular tissue. By the contraction of these the 

 thin wall of the auricle is made to compress the 

 blood contained in the auricular cavity, and 

 force it onwards into the ventricle. But an- 

 other network of still finer fibres, but not con- 

 tractile, will be seen ramifying upon and amongst 

 the muscular fibres. These are branches of 

 nerves, through the instrumentality of which the 

 contraction of the muscular tissue is effected. 



At the lower part of the drawing, to the right, 

 are seen some angular bodies of large size, 

 occupying the spaces between the muscular 



