THE MECHANICS OF RESPIRATION 325 



that the diaphragm is composed of a much more efficient muscular tissue 

 than that of the other muscles. 



The Effects of the Respiratory Movements on the Lungs. The changes 

 produced in the dimensions of the lungs by the inspiratory expansion of 

 the thoracic cavity are not uniform, since different parts of these struc- 

 tures are not equally extensible. From an anatomical standpoint, the 

 lungs may be divided into three zones: (1) The inner or root zone contain- 

 ing the bronchus, artery and vein, and their main subdivisions. The 

 large amount of fibrous tissue in this region offers great resistance to 

 any expanding force. (2) The intermediate zone, containing the vascular 

 and bronchial ramifications radiating towards the surface of the lungs, 

 with pulmonary tissue implanted between the rays. This part of the 

 lungs has varying degrees of extensibility, the pulmonary tissue having 

 the most and the vascular and bronchial the least. (3) The outer zone, 

 perhaps 25 to 30 mm. in depth, composed of pulmonary tissue and 

 equally extensible throughout (Keith 1 ). The expansion of the lung is 

 accomplished by a moving apart of the less extensible rays of tissue so 

 as to permit the expansion of the more extensible pulmonary tissue be- 

 tween them. Keith compares the mechanism to that seen in the opening 

 of a Japanese fan. 



Because the lung expands in the direction of least resistance, study 

 of the inflated dead lung does not reveal the normal expansion brought 

 about by the thoracic movements. In the living body expansion is more 

 limited in some regions than in others. Of the five areas which may be 

 distinguished on the surface of the lungs, three are in contact with rela- 

 tively immovable parts of the chest wall, and therefore can not be ex- 

 panded directly. These are: the mediastinal, in contact with the pericar- 

 dium and the structures of the mediastinum ; the dorsal surface, in contact 

 with the spinal column and the posterior aspect of the thoracic cage, and 

 the apical surface. The motions of the first pair of ribs and the manu- 

 brium expand chiefly the anterior and ventrolateral part of the apex 

 of the lung, and have only an indirect influence on the dorsal part of the 

 apex i. e., the part lying directly in front of the necks of the first and 

 second ribs, the most common site of pulmonary tuberculosis. The two 

 surfaces of the lungs which are directly expanded are the diaphragmatic 

 and the ventrolateral or sternocostal. Meltzer 4 found that the negative 

 pressure in the thorax during inspiration was least along the relatively 

 stationary walls of the thorax, and greatest in the regions nearest the 

 diaphragm. From this he concludes that some of the expanding force 

 is lost as it passes through the lungs to the surfaces of indirect expansion. 

 Many observers have claimed that the expansion of the lung does not 

 take place throughout instantaneously and equally. This is illustrated 



