CHAP. XXIX.] MOVEMENTS OF THE THORAX. 



397 



Fig. 209 





are added other muscles, which may enlarge or diminish the; area 

 of the cavity, and thus act in respiration, but which are, for the 

 most part, subservient to the general muscular movements of the 

 body. 



The thorax is capable of enlargement in all its dimensions, in 

 height, depth, width. Its vertical extent is increased by the eleva- 

 tion of the ribs and the widening of the intercostal spaces, but 

 chiefly by the descent of the diaphragm. Its antero-posterior and 

 transverse diameters are increased by the elevation of the ribs, 

 which carry forwards as well as raise the sternum (and the lower 

 end of that bone usually a little more than the upper, in conse- 

 quence of the greater length and obliquity of the lower sternal ribs), 

 and which also seem to 

 undergo a slight rotation 

 on a line joining their 

 two extremities, by which 

 their middle part is raised 

 and slightly removed from 

 the median plane of the 

 thorax. These are the 

 principal modes provided 

 in the mechanism of the 

 thoracic walls, for its dila- 

 tation in breathing. It 

 should be added that 

 during the advance of the 

 sternum the arch of the 

 ribs is widened, chiefly by 

 a torsion of their carti- 

 lages, and that the elastic 

 rebound of those parts is 

 a powerful agent in ex- 

 piration. 



Observed Movements. 

 All the foregoing modes 

 of enlargement of the 

 thorax act in deep in- 



; 



Diagrams showing the extent of antero-posterior movement 

 in ordinary, and forced respiration in male and female. " The 

 back is supposed to be fixed, in order to throw forward the 

 movement as much as possible." The black line indicates, by 

 its two margins, the limits of ordinary inspiration and expira- 



TVII+ in /vf/li'-naY.T'- tion * In f orced inspiration, the body comes up to the dotted 

 , UUC 111 oruindl^ line, while in forced expiration, it recedes to the smallest space 



breathing, there are con- j 



siderable differences according to sex and age. In men an ordinary 

 inspiration is attended with very slight elevation of the ribs, not 

 more than one-twentieth of an inch, according to Dr. Hutchinson's 



