140 ANIMAL PHYSIOLOGY. 



tion these are further elevated by the first pair of ribs being 

 themselves raised by muscles descending from the neck. The 

 elevation of the first pair of ribs increases the capacity of the 

 thorax considerably; for although those ribs are short, and 

 embrace but a small portion of lung, their elevation involves 

 the additional raising of those below. 



If a finger be placed on the twelfth rib during the fullest 

 inspiration, it will be found that it is scarcely, if at all, 

 raised, that its principal movement is backwards; and the 

 importance of this will be appreciated when it is considered 

 that the thoracic cavity is increased in size by depression of 

 its floor, and would be diminished rather than enlarged by 

 elevation of the attachments of the diaphragm. In fact, the 

 the four lowest pairs of ribs are specially acted on in inspira- 

 tion by a pair of muscles (serrati postici inferiores), whose 

 office is to pull them backwards and resist their elevation. 

 The backward enlargement of the thorax, particularly in its 

 lower part, is further provided for by the downward and 

 backward direction taken by the ribs as they extend out 

 from the back bone, and is exceedingly important, as it is at 

 the lower and back part of the thorax that the greatest mass 

 of the lungs is situated. That is a fact which ought to be 

 generally borne in mind, not only as pointing out one source 

 of evil from the vicious habit of tight-lacing, but because it is 

 a popular error that in protecting the chest from cold, it is 

 sufficient to accumulate warmth on the fore part. Bronchitis, 

 no doubt, is liable to have its source in exposure to cold 

 by the mouth or in front of the chest, but pneumonia or 

 inflammation of the texture of the lung is more frequently 

 traceable to exposure of the waist; for example, by the 

 evaporation of a shirt damp from perspiration. 



102. The muscles by which the ribs are moved are placed 

 between them, forming two layers, sloped in opposite direc- 

 tions, and named the external and internal intercostal 

 muscles.* In full inspiration muscles of the neck are also 



* On the Continent it seems to be now generally admitted that 

 both these sets of fibres are of service in inspiration ; but in English 

 text-books another theory continues to hold its ground, which is only 

 thus referred to that the student may be warned against it as an 

 error. The merits of the question cannot be here discussed. 



