78 PROFESSOR DAVID HEPBURN ON 



appears to me, therefore, that the double mechanism which KEITH supports for the 

 expansion of the apical and basal lobes of the lung is more apparent than real, and that 

 the so-called types of breathing in man are rather the result of the erect attitude 

 whereby, from the natural configuration and diameters of the diaphragmatic section of 

 the thoracic cavity in the two sexes, it is with less muscular effort that the male expands 

 the lower part of his chest and the female expands the upper part of her chest in order 

 that each may obtain the amount of air necessary for ordinary quiet breathing. When 

 additional efforts call for more air, or when, as in the supine position, the easy movement 

 of the chest is impeded, then there is an immediate departure from the characteristic 

 method ; but I do not think that we must postulate a double inspiratory mechanism. 



The key to the whole mechanism of inspiration is undoubtedly the part performed by 

 the contraction of the diaphragm. We cannot, therefore, overestimate the importance 

 of its attachments and structure ; nor must we forget that, like any other muscle, its 

 action is the result of the contraction of its fibres, whereby its attached ends are brought 

 more or less near to each other. The most favourable method of examining the diaphragm 

 is to consider the adult structure from the point of view of its development. 



The first part of the diaphragm to appear in the embryo, and the part which may 

 be considered the most powerful in the adult, is its mesial or vertebro-sternal portion, 

 whose vertebral ends or crura arise from the lumbar vertebrae and constitute its axial 

 or fixed end. These muscular fibres having adapted themselves to the positions of the 

 abdominal aorta and the oesophagus, by a certain amount of intermingling of the fibres 

 from opposite sides of the mesial plane, become inserted into the central tendon, whose 

 shape varies from the trefoil tendon of man to the vena caval ring with its lateral septa 

 as seen in the Weddell seal. From the ventral aspect of these tendinous structures a 

 second set of muscular fibres extends to the deep surface of the lower or hinder end of 

 the sternum. The mesial part of the diaphragm is therefore in reality a digastric 

 muscle pursuing an arched course from the vertebral column to the sternum. The 

 arched character of its course is more pronounced in its dorsal segment, while in its 

 ventral or sternal segment the arched character is lost, being replaced by a straight or 

 flat course. This change in the curve of the two segments is due partly to the disposi- 

 tion of the abdominal viscera and partly because the vertebral attachments are some 

 distance farther tailwards than a point which would correspond with the hinder end of 

 the sternum. 



By its contraction two results may follow : ( 1 ) the lower (hinder) end of the sternum 

 is either drawn closer to the vertebral column or else prevented from being projected 

 ventrally (forwards) by other influences ; (2) the arched dorsal segment between the 

 vertebral column and the central tendon becomes more or less flattened, and in conse- 

 quence the adjacent abdominal viscera are pushed towards the ventral abdominal wall. 

 At the same time its restraining or bracing action upon the hinder end of the sternum 

 becomes correlated to the restraining action of the first costal arches upon the manubrium 

 sterni, whereby the manubrium sterni is maintained at a relatively fixed distance from 



(ROT. SOC. EDIN. TRANS., VOL. XLVIII., 328.) 



