118 RESPIRATION. L.ECT. VI. 



or less developed, and have diversities of form and struc- 

 ture, adapting them to the medium in which the animal 

 usually lives. 



In fishes, for example, the organ of respiration is a mu- 

 F - 8 cous membrane, provided with many 



folds, and divided into filaments, or 

 lamella?, and abounding in blood-ves- 

 sels. It is always in contact with the 

 water, which is introduced through 

 the mouth, and expelled by the 

 branchial fissures. In these animals 

 every thing is arranged so as to give 

 the greatest possible extent of sur- 

 face for the contact of the water, in 

 one of tbe arborescent pro- w hi c h the atmospheric air is dis- 



cesses, forming the gills of T 



Doris Johnstoni, separated Solved, With the Vascular Walls. In 



and enlarged. fa e Comm0 n ray, the branchiae or 



gills have a surface of 2250 square inches. 



In reptiles, birds, and mammals, the respiratory organ 

 consists of an expansion of the bronchial tubes, which ramify 

 like a tree, and whose very delicate extremities terminate 

 by a large number of spheroidal vesicles placed side by 

 side, and surrounded by small blood vessels. The respira- 

 tion of some reptiles, at least during the first period of their 

 existence, is both that of fishes and mammals: hence they 

 have at the same time both branchiae and lungs. 



Mechanism of Respiration. The movements necessary to 

 this function are partly voluntary, partly involuntary. They 

 may be reduced to two acts, one by which air is introduced, 

 another by which it is expelled. All the air passages dilate 

 during inspiration, all contract during expiration. The 

 combined action of the muscular force, and of the elasticity 

 of the osseous and cartilaginous parts of the thorax, as well 

 as of that which is peculiar to the walls of the air vesicles, 



