662 



RESPIRATION 



therefore normally stimuli are constantly pausing 

 up the vagi to the centre, ami accelerating the dis- 

 charge of impulses liy the centre. Still, an accelerat- 

 ing effect is not the same thing as an initiating 

 -limuliis. Further, since respiration goes on when 

 the higher purl- of the lirain are removed, impulses 

 proceeding from aliove are not essential ; anil since 

 v hen the spinal cord is cut lielow the medulla the 

 movements of the nostrils and vocal chords con- 

 tinue (although of course all others cease), the 

 centre works independently of sensory impulses 

 arriving from any nerve, except the rmiiinl nerve- ; 

 and since these cranial nerves may be divided, if 

 the medulla and spinal cord lie left intact, without 

 any effect upon the respiratory movements, we 

 may conclude that the centre is automatic in its 

 action, hut may be influenced from without. 



The more venous the blood the greater is the 

 activity of the centre ; when the blood reaches a 

 certain state of impurity convulsions arise. We 

 may conclude that the state of the blood affects 

 the centre directly, and not rellexly, by stimulat- 

 ing the endings of afferent nerves in various parts 

 of the body ; because if the supply of Mood lie cut 

 olf from the medulla alone the same effects are 

 produced. Venous blood differs from arterial 

 blood iii containing less oxygen and more carbonic 

 acid. The deficiency of oxygen is the cause of 

 the greater activity of the centre, because if an 

 animal breathe an atmosphere of nitrogen the 

 carbonic acid does not accumulate in the blood. 

 and yet convulsions occur ; whereas if the animal 

 breathe an atmosphere containing sufficient oxygen 

 but excess of carlranic acid, then the convulsions do 

 not occur, but the animal may become unconscious 

 through some of the higher centres lieing poisoned. 

 When in action the centre discharges motor im- 

 pulses down various nerves to all the muscles con- 

 cerned in the respiratory movements. If any of 

 the nerves lie cut the movements of the muscles 

 supplied of course cease, since they are no longer 

 stimulated by impulses proceeding from the centre. 



The. Chemistry of Respiration . We have now 

 to explain the pa age of oxygen from the air- 

 chambers of the lungs into the Mood that circulates 

 in the vessels of the chamlier-walls, and the pa age 

 of carbonic acid from the blood into the air within 

 the lungs. 



In order to understand what follows we shall 

 have to study the law- of diffusion (see also DIFFU- 

 SION ). A gas consists of a great nuinlier of separate 

 molecules moving with great speed. The numlier of 

 these moll-cull'- in a cubic inch of a gas ( at ordinary 

 temperature ami pie me i i- estimated about 10" 

 or 1,000,000,000,000,000,000,000. Each molecule is 

 so small that the space lietween adjacent mole- 

 cules is large compared with the size of the mole- 

 cnles ; therefore, each molecule during its move- 

 ment has a large path free from colli-ion with other 

 molecules. The average s|>eed of a molecule varies 

 with the temperature, increasing as the tempera- 

 ture rises. The molecules lying near the surface 

 of any mass of gas will constantly impinge UJKIII 

 the boundaries; these impacts are so numerous 

 and -o close that they produce an apparently 

 continuous /irrxxure all over the boundary. Tliis 

 pressure obviously dc|>ends only ii|>n the density 

 (nuinlM-r of molecules in unit-space) ami the tem- 

 perature (average speed of the molecules) of the 

 gas. Further, the molecules of a gas are so far 

 apart that when two or more gases are mixed their 

 molecules interfere HO little with each other that 

 each gas everts the same pressure upon the walls 

 of the containing vessel as it would do were it 

 alone present. In such a case the total pressure is 

 the sum of the t wo or more /mrtiat pressure* of the 

 everal gases. If the space in which agas is enclosed 

 be diminished the molecules are brought nearer to 



each other, until a point is readied at which many 

 of the molecules apparently act upon each other in 

 such a way as to become more complex moleculet, 

 tli in forming a liijuiil in the lower part of th 

 vessel with its gas in the upper part. The complex 

 molecules are still in motion, and interchange, or 

 iliffiixinn, constantly takes place between the two 

 regions. The numlier of molecules leaving the 

 gaseous region depends only upon the state (tem- 

 perature and density) of the gas. The number 

 leaving the liquid depends only upon the state 

 (temperature and density) of the liquid. When 

 the diffusion takes place in a closed space a state 

 of equilibrium of interchange is soon reached. In 

 the lungs the liquid molecules of the oxygen of the 

 blood are being constantly moved past the common 

 surface between the air and the blood ; the inflow 

 therefore of oxygen from the air into the blood is 

 greater than the outflow from the blood to the air . 

 On the other hand, the gaseous carlmnic acid in the 

 air is constantly removed from the common surface 

 between it and the blood ; and therefore the outflow 

 of carbonic acid from the blood into the air is 

 greater than the inflow from the air into the blood. 

 This picture of the state of matters that regulate 

 the interchange of gases in respiration is simpler 

 than the reality. The further complexity will be 

 descril>ed immediately. 



We must know what are the laws governing I he. 

 diffusion when the gas above the liquid is not the 

 gas of the liquid, as is the case when air rests upon 

 a surface of water. Some of the molecules of the 

 air will become entangled in the liquid, will form 

 the liquid of the particular gases within the 

 other liquid, and then the state of affairs will 

 lie as liefore, so far as the gases, and their 

 liquids, of the air are concernetl, and a state of 

 equilibrium between each of these gases and its 

 own liquid will lie formed. Hut now suppose 

 that the liquid and the gas have a special chem- 

 ical affinity for one another, as is the case with 

 the oxygen of the air and a sulistance in the blood, 

 and as is the case with the carbonic acid of the 

 Mood and a sulistance or substances in the blood. 

 As soon as the gas has diffused into the liquid the 

 chemical compound will be formed ; but now the 

 reverse effect will lie-in, dissociation of the nun. 

 pound will occur, but slowly, because a greater 

 violence of collision is necessary. Therefore, other 

 things lining equal, less pressure will lie needed to 

 maintain equilibrium, because fewer liquid mole- 

 cules of the compound will become gaseous, and 

 therefore fewer gaseous molecules need become 

 liquid to preserve equilibrium. Indeed, it is found 

 that at a certain temperature and a certain pressure 

 the dissociation scarcely takes place at all ; hut if 

 tempera) lire lie raised, or if the pressure be lowered 

 to a certain point, then the dissociation will be 

 very rapid. 



These laws of diffusion apply to the gases of the 

 blood. In the investigation of these gases a 

 sample of blood is placet! under the receiver of an 

 air-pump (thus imitating, though exaggerating, 

 the normal pumping action of the chest- walls), the 

 gases extracted are passed through various solu- 

 tions which retain the several gases, and thus they 

 may be estimated and examined. The quantity of 

 oxygen obtained from arterial blood is greater tlian 

 that obtained from venous blood. The arterial 

 Mood of a. dog yields for every 100 vols. at ordinary 

 pre-sure and C. 58 '3 vols. of mixed gases when 

 the external pressure is reduced to zero. This 

 mixture is composed of 23'2 vols. of oxygen, 34'3 

 vols. of carlmnic acid, and I 'H vols. of nitrogen. 



If blood took up a-s much of these gases b\ mere 

 dillusion as water does, it would contain O'HIi vols. 

 of oxygen, 1 U vols. of carlmnic acid, and 1 '6 vols. 

 of nitrogen. Therefore it is evident that, while 



