448 



RESPIRATION 



duce an inflow and outflow of air. Instead, it is to be clearly under- 

 stood that the size and capacity of this organ are varied by an outside 

 force which is applied to its entire external surface. This force is de- 

 pendent upon the activity of certain muscles, classified as respiratory 

 muscles, the sole function of which is to produce an enlargement of the 

 thorax and, in an indirect way, also of the lung. Consequently, the 

 expansion of this organ is a passive phenomenon as far as the lung is 

 concerned, but active as far as the muscles are concerned. This phase 

 is soon followed by a decrease in the size of this particular part of 

 the body and a decrease in the size of the lung. The former period is 

 known as inspiration and the latter as expiration. 



The principle involved in this process is well illustrated by the 

 behavior of the air-sacs of the insects. In these animals we find a 

 branched system of tubes which communicate through narrow orifices, 



known as stigmse, with small 

 saccules suspended in the body 

 cavity. On observing an insect 

 it will be seen that the volume 

 of its trunk is rapidly changed 

 from moment to moment. The 

 walls of its body are moved out- 

 ward by muscular force, the air- 

 sacs are expanded and air rushes 

 through the stigmse into their 

 interior. At this time, therefore, 

 the pressure within is lower than 

 without. Toward the end of in- 

 spiration an equilibrium is slowly 

 established which causes a cessa- 

 tion of the influx of air. The 

 expiratory movement now sets in. The body wall moves into its 

 former position largely by recoil with the result that the air in the 

 saccular spaces is subjected to a pressure higher than that of the 

 atmosphere. The air now escapes through the stigmse until an equali- 

 zation of the pressures has again been attained. This alternate ex- 

 pansion and compression of these air-spaces enables them to obtain a 

 constant supply of fresh air by means of which the partial pressures 

 and the diffusion of the gases may be kept up indefinitely. 



Special Respiratory Organs. — Animals may be divided into two 

 classes, namely, into those living in atmospheric air and those living 

 in water. Accordingly, two types of respiratory organs have been de- 

 veloped, namely, the lungs and the gills, the latter being the phylo- 

 genetically older mechanism. Moreover, those animals which spend 

 their life in part in the former medium and in part in the latter, are 

 in possession of lungs as well as gills. It is true, however, that these 

 organs are generally not functional at the same time, because the change 

 of an aquatic into a terrestrial animal is usually associated with a 

 gradual atrophy of the gills. 



Fig. 227. — Diagram of an Elementaet 

 Lung. 

 S, stigma; O, oxygen diffusing from air of 

 saccule into tissue fluids; CO2, diffusing in re- 

 verse direction. 



