328 PROFESSOR DAVID HEPBURN ON 
appears to me, therefore, that the double mechanism which Keir 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 diaphre 
is to consider the adult structure from the point of view of its development. 9 
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 vertebree and constitute its axial 
or fixed end. These muscular fibres having adapted themselves to the positions of the 
abdominal aorta and the cesophagus, 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 
