ANIMAL STRUCTURE AND FUNCTION. 'It 



tense or slack, thin or thick according to the strain placed upon them, 

 and as the distance between them is widened or narrowed at the same 

 time, the rush of air through this constantly varying aperture produces 

 the sounds of the voice. If they are stretched to their utmost, and 

 brought close together, a high note results, while relaxation and 

 separation of their edges gives a deep tone. 



The windpipe extends from the larynx to the lungs ; it runs along the The 

 lower border of the neck, and can be easily felt as far down as the windpipe, 

 entrance to the chest. It is composed of rings of cartilage (gristle), 

 the ends of which overlap each other, and as this arrangement permits it 

 to withstand considerable pressure, it is not often injured. It is lined by 

 microscopic hairs, constantly agitated in an upward direction, by means 

 of which phlegm is brought up from the lungs. At the entrance to the 

 chest the windpipe divides, one branch going to each lung, and from this 

 on, they divide and divide until they become very minute, when they 

 lose their gristly rings, and become thin membranous tubes, which 

 continue to divide in the same fashion. The ultimate divisions expand 

 suddenly into little, mulberry shaped balloons [air vesicles)^ the walls of 

 which are so thin that they allow the gases of the air and blood to pass 

 freely from one to the other. 



The Limgs {lights) are two large, elastic organs which, except for the The lungs 

 space occupied by the heart, completely fill the chest. They may be (lights), 

 described as equally full of air and blood, the entire space, not 

 occupied by the air vesicles just described, being filled with blood-vessels. 



The outside of the lungs, and the inside of the chest, is covered by a 

 smooth, shiny, slippery membrane {pleura)^ which prevents friction. 



The Diaphragm {inidrijf) is a strong, thin curtain of muscle which The dia- 

 separates the chest from the belly ; it is attached to the inner sides of the P^'"^?^ 

 ribs, beginning high up under the backbone, just in front of the loins, and (m'<i"")- 

 slopes downwards and forwards to the breastbone. It is pierced by the 

 great blood-vessels, and the gullet as they pass out of the chest ; its 

 front is covered by pleura, and lies in touch with the lungs, whilst the 

 liver and stomach are immediately on its other side. 



Chajiges which take place dm'ing respiratio?!. — The blood which is Changes 

 sent out to the body by the arteries (arterial blood) is bright scarlet, and which 

 full of oxygen from the fresh air, .but after circulating through the various take place 

 organs, and supplying their needs, it loses some of its oxygen, receives <^""9g 

 some carbonic acid gas in exchange, and in consequence acquires a ^^^ '^^" 

 deeper red colour. In this condition it returns by the veins (venous 

 blood), and is sent to the lungs. Arrived there, it^ circulates round the 

 walls of the air vesicles, and another interchange of gases takes place, 



