

RESPIRATION. 117 



ceived the name, from anatomists, of the wonderful net-work ; while 

 the air received into and expelled from the lungs, and consequently 

 brought into contact with its air-tubes and blood-vessels, cannot 

 be less, in an ordinary man, than between three and four thousand 

 gallons daily. 



Fresh supplies of air, then, that the blood may be purified, are 

 the essential objects of a respiratory apparatus ; and from the 

 necessity of having some modification of such an apparatus, no 

 animal whatever is exempted, although the supply of air required 

 varies much. A frog or lizard, for example, will live a considerable 

 time in air which a bird has been forced to breathe till it has died, 

 and insects will live for a long period even in the air that has 

 ceased to support both the bird and the lizard. Fishes, again, 

 whose gills perform the same office as our lungs, can exist upon 

 the small portion of air they extract from the water in which they 

 swim. But, however small the quantity required, none can want 

 it altogether ; and if any of them be placed under the receiver of an 

 air-pump, and the air be exhausted, they immediately become dis- 

 tressed, and die in a short time. 



There is one remarkable circumstance that may be noted when 

 the motions of the heart, or intestines, and those of respiration, 

 are contrasted. The motions of the former are entirely removed 

 from the influence of the will, and usually do not excite in us any 

 consciousness of their existence ; while those of respiration are 

 always preceded by a sensation, if not also by an act, of volition. 

 Before the air is drawn into the chest, we have always a peculiar 

 sensation, reminding us that a fresh supply of this material is re- 

 quired. At first, this sensation is merely a gentle intimation ; but, 

 if neglected, it becomes so intolerably painful, as to compel us to 

 relieve it by breathing. When an individual becomes partly insen- 

 sible, the sensation requires to be considerable before he attends to 

 it ; and accordingly we find, that, instead of breathing, as we ordi- 

 narily do, fifteen or twenty times in a minute, he will breathe only 

 once in half a minute. When insensibility increases still further, 

 this and all other feelings become extinct, and then he dies. It is 

 upon this principle we give an explanation of sighing. When a 

 perso'n sighs, the mind has been intensely fixed on some object. 

 The consequence is, as Dr. Darwin has remarked, that he forgets 

 for a short time to breathe, until the sensation in the chest becomes 

 so importunate as to oblige him to make a more than usually full 

 inspiration to relieve it. 



