CHAPTER LII. 



THE RESPIRATORY SYSTEM 



It will be remembered that in studying the frog, the trachea and 

 oesophagus have their beginning close together at the caudal end of the 

 pharynx which is also the beginning of the cephalic end of the larynx. 

 In the higher forms of animals the trachea divides into two bronchii. 

 These bronchii again continually subdivide until there are many tiny 

 tubules called bronchioles spreading out to all parts of the lungs. These 

 bronchioles form a sort of an air-capillary system through which the 

 inspired air is sent to all parts of the lungs, there to assist in aerating 

 the entire pulmonary blood which has been sent to the lungs from the 

 heart, through the pulmonary artery. In order that the oxygen in the 

 inspired air can come in direct contact with the blood itself, there must 

 be a rather thin, more or less porous, membrane separating the blood 

 and air. 



The lungs, liver, spleen, and kidneys are known as parenchymatous 

 organs. It is well to bear this in mind constantly, for many diseases 

 find their way from one of these organs to another. A parenchymatous. 

 organ is more or less sponge-like and consists of loosely woven tissue 

 in which there are many porous openings. Such organs are invariably 

 supplied with great quantities of blood. 



These organs, especially the lungs, have a decidedly thin membrane 

 surrounding the sac-like ends of the bronchioles. Here the oxygen 

 passes through the thin walls to come into direct contact with the venous 

 blood which has been sent there through the pulmonary artery. 



What has been said so far regarding the respiratory system applies 

 to vertebrates at large, although those who live a part of their lives in 

 water, have no lungs during that period, and in this respect resemble fish 

 and other animals which spend all of their time in the water. In such 

 forms gills (also called branchiae) develop on the walls of some of the 

 visceral clefts (these are also called gill clefts (Fig.. 295) or branchial 

 clefts). The clefts come from the sides of the pharynx and begin as a 

 pair of pouches or grooves of the pharyngeal entoderm. Extending 

 toward the sides of the animal they push aside the mesoderm until they 

 reach the ectoderm. The ectoderm and entoderm then fuse to form a 

 plate. This plate becomes perforated, thus connecting the pharynx with 

 the exterior of the body by a number of openings. These openings or 

 clefts begin development at the cephalic end and successively continue 

 caudad. 



The visceral pouches, although developing in all verteberates, do 

 not as a rule break through in the mammals. In fact, the pouches may 



