THORAX. 



1065 



established an experimental inquiry into the 

 quantity of air received by a single inspiration.* 

 Jurin improves upon Borelli. About this 

 time (1708) Dr. James Keile made some 

 correct measurements of the volume of air 

 breathed, f Then followed Dr. Hales, who 

 threw more light upon the doctrine of air, 

 the power of respiration, and the power of the 

 heart, than all his predecessors ; yet he was 

 quite ignorant of the use of respiration ; and 

 at this period (1733) really very little was 

 known upon the subject. In 1757 and fol- 

 lowing years, Black, Rutherford, Lavoisier, 

 Priestley, and Scheele, the chemists of the 

 age, threw light upon the matter by discover- 

 ing the composition of the atmosphere, and 

 consequently the composition of respired air. 

 It is since the time of Black that the most 

 valuable mass of our knowledge upon respir- 

 ation has been discovered. 



The functions of the thorax maybe divided 

 into two great heads, the chemical and the 

 physical ; for an account of the former see 

 RESPIRATION. 



Every point of the thorax can move for the 

 purpose of respiration; and hencehas followed 

 a. division of these movements, nominated 

 after the parts which respectively carry on 

 their functions, viz. costal breathing, and ab- 

 dominal or diaphragmic breathing. These 

 motions are, in health, symmetrical, constant, 

 regular, sensitive, and precisely the same ; 

 otherwise disease must exist. 



The breathing movements are also expres- 

 sive of mental emotions. The tragedian imi- 

 tates them to give force to the character 

 he represents, whether it be the stealthy 

 breathing of the Roman conspirator sharpen- 

 ing his knife, or the deep swelling inspiration 

 of Ajax defying the lightning; these repre- 

 sentations, without such movements, would 

 be but dull pictures of the mind of the 

 authors who left such characters on record. 

 It is much easier to become delicately fa- 

 miliar with these movements and their cha- 

 racteristic differences, than it is to describe 

 them. 



The latitude of movement, performed by the 

 walls and floor of the thorax, admits of three 

 common degrees of division : 



First, extreme expansion or enlargement. 



Second, extreme contraction or diminution ; 

 and 



Third, an intermediate condition, an or- 

 dinary or quiescent state. 



These three divisions necessarily displace 

 volumes of air of different magnitude. 



Were the respiratory movements but two 

 in number, extreme expansion and extreme 

 contraction, the quantity of air moved, and the 

 character of the movement, would be easy of 

 calculation and expression; but the interme- 

 diate breathing or quiescent movement being 

 so limited, so perfectly under the control of 

 the will, so readily affected by mental emo- 

 tions and by the animal functions, renders 



* De Motu Animaliiun, p. 2. prop 81. 

 f Tentaui. Med. Phys. p. 80. 



the calculation of the volume of air ordinarily 

 passing through the lungs a very difficult 

 question. 



The quantity of air in the thorax, together 

 with those portions which can be added at will, 

 may be arranged and denominated thus : 



First, residual air. 



Second, reserve air. 



Third, breathing air. 



Fourth, complemental air. 



Fifth, vital capacity. 



1st. Of residual air. After death the 

 lungs contain air, which is not displaced 

 by the last expiration ; this quantity remains 

 in the thorax as long as the lungs maintain 

 their natural structure; therefore we have no 

 control over this volume of air : to it we 

 assign the term " residual air" 



2nd. Reserve air is that portion which re- 

 mains in the chest after the gentle ordinary 

 expiration, but which may be displaced at 

 will. 



3rd. Breathing air is that volume which is 

 displaced by the constant gentle inspiration 

 and expiration. 



4th. The complemental air is that volume 

 which can at will be drawn into the lungs by a 

 violent exertion above the moderate effort of 

 ordinary breathing, constituting the deepest 

 possible inspiration. It is only occasionally 

 demanded. 



5th. The vital capacity is these last three 

 divisions combined, being the greatest vo- 

 luntary expiration, following the deepest inspi- 

 ration.* 



This division of thoracic movements for 

 commanding these different volumes of air 

 may be more clearly illustrated by diagram 703. 



Fig. 703. 



rCornplcmontnl air. 



Vital 

 capacity, { Breathing air. 



city. { 



l_ Reserve air. 

 Residual air. 



The division of the thoracic movements. 



Let that portion marked H represent the resi- 

 dual volume, or air left in the lungs, after a 

 complete voluntary expiration ; the part next 



* According to physiological nomenclature, 

 perhaps the term "vital capacity" may be objec- 

 tionable; but we adopt it, for want of a better term, 

 to signify a capacity or volume of air which can 

 only be displaced by living movements, and may 

 therefore he termed a " living volume," or " vital 

 capacity." 



