lo Oct., 1908]. Elements of Animal Physiology. 635 



THE ELEMENTS OF ANIMAL PHYSIOLOGY. 



{Coiitinued from page 528). 



W. A. Osborne. M.B., D.Sc, Professor of Physiology and Histology,. 

 Dean of the Faeiiltv of Agrieulture in the University of Melbourne. 



XIII. Respiration. 



No physiological activity is so obvious as that rhythm of movement 

 which we call respiration or breathing. The necessity that exists for 

 breathing is made up of a number of factors of which the following are 

 the most important : — 



1. The blood as it passes through the lungs takes up a supply of 

 oxygen from the air and gives off some of its carbon-dioxide. 



2. The flow of blood towards the heart in the larger veins is facilitated 

 by the squeezing action on the abdomen, and the suction in the chest, 

 which occur during inspiration. 



3. In most animals, and particularly those that sweat little, the lungs- 

 can get rid of some superfluous heat. 



The second and third factors have already been mentioned ih previous, 

 chapters; the first and most important action will be considered in this- 

 chapter. 



THE PHYSICAL FEATURES OF RESPIRATION. 



Air, which is destined to enter the lungs, enters by the nostrils and 

 then through the nasal passages into the pharynx. In yawning or violent 

 breathing air may be admitted by the mouth. From the pharynx the air 

 passes through a narrow opening called the glottis into the larynx 

 which latter may be looked upon as the first portion of the wind-pipe. The 

 aperture of the glottis is controllable ; it can be completely shut or 

 widened by means of special muscles. The trachea or wind-pipe is a 

 tube kept permanently open by means of incomplete rings of cartilage. 

 The trachea as it enters the thorax divides into two bronchi, one for each 

 lung, and these bronchi divide and sub-divi'de until an immense number of 

 bronchioles are formed like the twigs on a tree. Each bronchiole ends 

 in a small air-chamber with pouches or alcoves, called air-cells, around 

 all its sides. In the walls of these air-cells, and separated from the air 

 only by a very thin lining sheet of flat cells, is a dense mesh- work of 

 capillary vessels through which the blood, pumped from the right ventricle, 

 is constantlv flowing. Each lung can therefore be looked on as a complex 

 bag sub-divided into a large number of compartments. Thanks to this 

 sub-division the air, which is breathed in, can be brought into contact with 

 an immensely greater number of capillaries than if the bag were a simple 

 one. 



The outer surface of each lung is lined by an air-tight sheet of fibrous 

 tissue called pleura ; the inner surface of the wall of the thorax is simi- 

 larly lined. Now these pleural surfaces are practically in contact being 

 separated only by a thin layer of lubricating fluid. The lungs are highly 

 elastic, and, in no matter what position the thorax is, are always in a state of 

 stretch. Thus if the chest wall is opened, as may occur through a wound, 

 and air is admitted from outside, or if the lung and its lining of pleura 

 be ruptured so that the air within can escape outwards, the lung on the 



