330 PROFESSOR DAVID HEPBURN ON 
by Macatister* as strong and rounded. A similar ligament of weaker character 
attaches the pericardium to the ensiform cartilage. These ligaments, especially the 
former, would resist the traction of the pericardium in the abdominal direction, and so — 
resist the depression of the central tendon. Again, the normal liver presents indented 
grooves corresponding to, and resulting from apposition with, the ribs which cover it, 
and these grooves indicate a fairly constant relation between the liver and the ribs, 
since they could not be formed by any plunging or piston action communicated to the 
liver from the diaphragm. If, on the other hand, the flattening of the dorsal portion of 
the diaphragm pushed the liver ventrally towards the ribs, and at the same time the 
contraction of the ventro-lateral portions of the diaphragm drew the lower ribs towards 
the liver or even maintained them in a position to resist the liver, then the liver mark- 
ings would be at once accounted for. Further, Sraruinet states that during ordinary 
respiration the central tendon of the diaphragm is practically motionless, but that as 
soon as respiration becomes laboured there is an actual downward movement of the 
diaphragm, and that during laboured inspiration the breathing is mainly thoracic in 
both sexes, ‘‘and the abdomen recedes with each inspiration.” Apparently, therefore, 
according to this observer, it is fair to conclude that a laboured inspiration, by calling 
for more powerful action of the diaphragm, results not only in greater flattening and 
depression of the dorsal segment of the arched diaphragm, but also in the lower ribs 
being drawn closer to its central tendon than during ordinary breathing, which is just 
what an examination of the muscular attachments would lead one to expect. I there- 
fore arrive at the general conclusion that the diaphragm is the keystone in the inspira- 
tory mechanism, and that its chief action consists in resisting the ventral (forward) 
movement of the hinder (lower) end of the sternum and of those ribs which are not 
directly articulated to the sternum. As a result of this controlling action the whole 
series of ribs may participate uniformly in a general lifting or elevating movement 
which characterises their position at the end of inspiration as contrasted with their 
sunken or depressed position at the end of expiration. Such an elevation of the ribs 
does not call for any rotation of their shafts, and indeed, from the nature of their 
capitular and tubercular articulations with the vertebral column, rotation of the shaft 
would be impossible. But the chondro-sternal as well as the articulations just 
mentioned are from the nature of their ligaments adapted to the movements of eleva- 
tion and depression of the rib as a whole, and even a small amount of such movement | 
at the vertebral end of a rib would tend to be magnified by the length and obliquity 
of its shaft. In fact, the capsules of the costo-transverse articulations are sufficiently 
long to permit of the gliding action necessarily associated with such elevation and 
depression. 
In the seal there are two muscles whose attachments are only readily comprehen- 
sible when considered as part of the mechanism for depressing the ribs after they have 
* MAcALISTER, Text-book of Human Anatomy, 1889. 
+ E. H. Staruine in Schafer’s Text-book of Physiology, vol. ii. pp. 276 and 280. 
