292 PULMONARY MUCOUS TISSUE. 



not be done as well by the fibrous tissue. The presence of 

 the muscular tissue is explained, if we remember the general 

 properties of muscle, which is the most elastic tissue of 

 the whole economy. In this case a tissue of peculiar elas- 

 ticity is required, the dimensions of the intercostal spaces 

 changing incessantly in movements of the thorax. A tissue 

 was required which would remain tense between the ribs, 

 which would not be depressed from without inwards by exte- 

 rior pressure during inspiration, or from within outwards by 

 intrapulmonary pressure during expiration. This function is 

 so important that, in order to fulfil it, the elasticity of the 

 muscular tissue of the intercostal muscles must be kept in 

 constant repair by nutrition ; for instance, if, in pleuritis, 

 inflammation has extended to these muscles, they become 

 powerless to perform their appointed function, and in such 

 cases an autopsy shows the lungs transversely grooved, 

 having received this impression from the intercostal spaces, 

 which then become capable of making this depression. 



The necessity of preserving a constant elasticity of the 

 intercostal spaces explains, finally, the existence of two layers 

 of muscles, the external and the internal intercostal muscles. 

 A simple diagram of the direction of these muscles (called 

 Hamberger's diagram, Fig. 78) shows that, as the ribs de- 

 scend (in expiration), the distance between the points of 

 insertion of the intercostal muscles increases ; and, again, 

 diminishes as they rise (in inspiration) : the reverse takes 

 place in the case of the internal intercostal muscles. From 

 this fact conclusions have been drawn as to the effect pro- 

 duced by the contraction of these muscles, the external 

 being considered as elevating or inspiratory muscles, and the 

 internal as depressing or expiratory (Hamberger). This 

 diagram may be more clearly explained, however, it seems 

 to us, by saying that the elasticity of the external intercostal 

 muscles is brought into play during expiration, and that of 

 the internal intercostal during inspiration. This alternation 

 of elasticity in the wall is quite indispensable ; because, in 

 inspiration it is depressed from without inwards, and in 



tion, and perform the office of a resisting wall: Van Helmont, 

 Arantius, Cruveilhier: rather, they contract, not to produce the 

 movements of inspiration and expiration, but in order, when they 

 do occur, to resist the pressure of either the exterior or interior 

 air (Kiiss). (See Aug. Jobelin, " Etude Critique sur les Muscles 

 Intercostaux." These de Strasbourg, 1870, No. 287.) 



