6 TEXT-BOOK OF ZOOLOGY 



and pressure the skin. From these organs the stimuli are conveyed by 

 nerves to the brain, where they are manifested in our consciousness. 

 Thus, the eye, the ear, the tongue, and the skin are the instruments of 

 the senses (of sight, hearing, etc.), or, in other words, implements or 

 organs of sense. In the vertebrata these organs exhibit a highly-perfected 

 development. 



5. Respiration. No animal can continue to exist if its respiration is 

 for any length of time interrupted i.e., if it is prevented from taking in 

 fresh air. Nor is the kind of air so taken up to be left out of considera- 

 tion, for any animal will speedily die of suffocation in an air not con- 

 taining oxygen gas. If we examine the air which has been breathed 

 out (e.g., by man), we shall find that a portion of the oxygen has 

 disappeared (prove this by showing that a flame burning in a vessel 

 containing such air is extinguished much sooner than when burning in a 

 vessel filled with atmospheric air), and this portion of oxygen has been 

 replaced by carbonic acid gas. (Prove by breathing into lime-water.) 

 Hence, respiration consists in the consumption of oxygen and in the 

 production of carbonic acid gas, and is accordingly a kind of combustion. 

 The exchange of the two gases is effected either by means of lungs or 

 gills, according as the animal derives its oxygen direct from the atmo- 

 sphere or from atmospheric air dissolved in water. 



Let us explain this process of respiration more fully in a mammalian 

 animal breathing by means of lungs. The air is taken in through the 

 nose (or mouth), and conducted through the trachea to the huiys. (The 

 upper part of the trachea is formed by the larynx, which is protected 

 against the entrance of food by means of a lid or cover. Two bands 

 vocal chords are stretched across the larynx, and are made to vibrate 

 by means of currents of air of sufficient strength issuing from the lungs. 

 Tones are thus produced in exactly the same manner as in a reed-pipe.) 

 The tracheal tubes ramify in the lungs like the branches of a tree, and 

 terminate in a very large number of very minute air-cells, the pulmonary 

 vesicles. Each of these extremely fine air-cells is surrounded by a net- 

 work of bloodvessels of hair-like fineness (capillaries}, which contain 

 blood very rich in carbonic acid. Two kinds of air, oxygen and carbonic 

 acid gas, are thus separated by the fine wall of these air-bladders. Now, 

 from an easily performed experiment, we learn that if two chambers 

 which are separated by an animal membrane are filled with different 

 gases, an exchange of the latter takes place through the membrane until 

 the two gases become completely mingled. In the same manner a mutual 

 exchange between the oxygen and carbonic acid gas takes place in the pulmonary 

 air-cells. The carbonic acid gas is expired, while the oxygen is taken up 

 and carried off by the blood. 



