CHANGES IN AIR AND BLOOD IN RESPIRATION. 669 



Systems of ventilation which have held in view simply the 

 object of maintaining the air at an approximately normal com- 

 position as regards the oxygen and the carbon dioxid have not 

 proved entirely satisfactory in practical use, and probably for the 

 reason that they have neglected to take into account the conditions 

 as regards temperature and moisture. Laboratory experiments 

 tend to show that individuals in a confined space may rebreathe 

 air until its composition is noticeably altered in regard to the 

 carbon dioxid and oxygen, and.,yet no distress be felt if provision 

 is made for avoiding a rise in temperature and humidity, and, on 

 the other hand, rooms may seem to be poorly ventilated, as judged 

 by the sensations, when the renewal of air is sufficient to prevent 

 an obvious change in its gaseous composition. 



The Gases of the Blood. The gases that are contained in the 

 blood are oxygen, carbon dioxid, and nitrogen. These gases may 

 be extracted completely and in a condition for quantitative analysis 

 by means of some form of gas-pump. The principle of most of the 

 gas-pumps used in the physiological laboratories is the same. The 

 apparatus is arranged so that the blood to be examined is brought 

 into a vacuum while kept at the temperature of the body. Under 

 these conditions all of the oxygen and nitrogen and part of the car- 

 bon dioxid are given off and may be collected by suitable means. 

 A portion of the carbon dioxid present in the blood is in such stable 

 combination that to remove it it may be necessary to add some 

 dilute acid, such as phosphoric acid. This portion of the carbon 

 dioxid is designated in this connection as the fixed carbon dioxid. 



The principle of the gas pump may be explained most easily by describing 

 the simple form devised by Grehant. The essential parts of this pump are 

 represented in Figure 272. The mercury pump consists of two bulbs, one 

 movable (M), the other fixed (F) . M may be raised and lowered by the wind- 

 lass (P) . Above F, there is a three-way stopcock (m) by means of which the 

 chamber F may be put into communication with the outside air by way of C, 

 or with the bulb B, which is to contain the blood, or may be shut off com- 

 pletely. If M is raised so as to fill F entirely, and the stopcock m is shut off, 

 then on lowering M the mercury will flow into it, leaving a perfect vacuum 

 in F, since the distance between F and M is greater than the barometric 

 height. If the stopcock m is turned so as to throw F into communication 

 with B, the chamber of this latter is brought under the influence of the vac- 

 uum and any gases that it may contain will be distributed between B and 

 F. If stopcock m is again turned off and M is raised, the gases in F will be 

 condensed at its upper end, and by turning the stopcock m properly these 

 gases may be forced to the outside by way of C or may be collected, if de- 

 sired, in a burette filled with mercury and inverted over the opening from 

 F contained in the bottom of C. In performing an experiment the flask 

 B, which is to contain the blood, is connected with F, as shown in the figure, 

 all joints being protected from leakage by a seal of water outside, as shown 

 at h, which represents a piece of wide rubber tubing filled with water so as 

 to protect a joint between two pieces of glass tubing. B is next exhausted 

 completely by raising and lowering M a number of times in the way described 

 above until on throwing B into communication with a vacuum in F no further 



