446 THE RABBIT. PHYLUM CHORDATA 



RESPIRATORY ORGANS 



The glottis (p. 440) leads into a short larynx and this 

 into a longer trachea, or windpipe. Both trachea and larynx 

 are supported by cartilages, which partly represent the branchial 

 arches (p. 319). The cricoid cartilage is a complete ring, 

 but the thyroid and the tracheal rings are incomplete dorsally. 

 The larynx contains the vocal cords, small bands of muscle 

 attached to the arytenoid cartilages. Their vibration, the quality 

 of which is altered by variation in their tension, is caused by aii 

 moving past them, and produces the voice. The rabbit is remark- 

 ably silent. The trachea passes into the thorax, and there divides 

 into two bronchi. Each of these divides and subdivides into fine 

 bronchioles, and the mass of these makes the lung. The end of 

 each terminal bronchiole is expanded into a blind air sac or 

 infundibulum, and the walls of this are hollowed out into air 

 cells or alveoli. These are supplied with fine blood vessels, and the 

 whole lung is thus not a hollow sac like that of the frog, but a 

 spongy mass with a greatly increased internal surface ; in man 

 this is about thirty times the surface area of the body. 



The chest or thorax is a closed box whose side walls are formed 

 by the ribs with the muscles between them, and its hinder wall 

 by the diaphragm, which divides the main or pleuroperitoneal 

 coelom, parting two pleural cavities in front from a peritoneal 

 cavity behind (p. 420). The cavity of the thorax can be enlarged 

 from back to breast, by a contraction of the intercostal muscles 

 which move the ribs outwards (and in man upwards) ; and from 

 head to tail by the movement of the diaphragm, which at rest 

 is convex towards the chest, but flattens when it contracts, 

 thus increasing the size of the thorax. Since the pleural cavities 

 are closed, their enlargement lowers the pressure within them, 

 and thus the lungs, which are elastic and contain air at atmo- 

 spheric pressure, must expand also to equalise the pressure on 

 the two sides of their walls. The air in them thus in turn has its 

 pressure lowered, and since they are open to the atmosphere, 

 air moves in from outside to maintain equilibrium. When the 

 inspiratory muscles relax, air is driven out by the collapse of the 

 chest owing to the elasticity of the lungs, but this can be aided 

 by the contraction of certain other muscles, notably those of 

 the belly, which press the viscera against the diaphragm from 

 behind. After air has been taken into the lungs, oxygen which has 



