STOMACH OF A MOUSE. 229 



capillaries are exposed to the air by means of lobules or 

 lunglets, each of which has its own bronchial tube and 

 system of blood-vessels. Each lobule has a central cavity 

 surrounded by a solid plexus of blood-vessels, which is not 

 covered by any limiting membrane, but admits the air freely 

 between the meshes. 



In the lungs of Reptiles the respiratory surface is 

 formed by the walls of an individual internal cavity, with 

 thin membranous wall, and simple, smooth expanse, except 

 at the upper end, where the tracheal vessels enter, close 

 covered with a network of capillaries, and these ramify over 

 the surface, depressing it into sacculi or air-cells, each of 

 which has a capillary network of its own, very considerably 

 increasing the surface of blood-vessels exposed thus to the 

 air. 



THE GILL OF AN EEL 



will present a beautiful object, and show how this external 

 lung is adapted for life in another element than pure air. 

 The laminae are divided into minute leaflets, over each of 

 which the finest possible web of capillaries is traced, and 

 the strength of the muscular apparatus connected with each 

 arch of laminae renews the fluid necessary for their perfect 

 aeration, without the cilia, which is needful in the gills of 

 Oysters, Mussels and Molluscs, also in the temporary gills 

 of the young Water-newt. 



THE INJECTED FIN OF A TURTLE 



is an exquisite object, showing the blood-vessels like coral- 

 branches gleaming in the water depths, less numerous, and 

 therefore more distinct in form, than in most circulatory 

 preparations. 



THE STOMACH OF A MOUSE. 



This is the best object for examination of and under-: 

 standing the structure of the stomach, as the section gives 

 a transparent view of the gastric vessels and also of the 

 pyloric opening. 



The animal stomach is a strong muscular sac, consisting 



