THE ANATOMY OF THE WEDDELL SEAL. 329 
the vertebral column. By reason of the somewhat rigid character of the first costal 
cartilages in man, as well as from the fact that they are frequently encased in an ossified 
shell, the restraining nature of the connection between the first pair of ribs and the 
sternum is very well marked ; but even in the Weddell seal, where a short and powerful 
fibrous ligament takes the place of the first costal cartilage, the manubrium is very 
firmly retained in its relation to the backbone. 
The next part to be added to the diaphragm developmentally constitutes its ventro- 
lateral seoments. In the adult these are composed of muscular fibres which arise from 
the ventral and lateral aspects of the central tendon. From this position they extend 
in a fan-shaped manner to be inserted into the deep surfaces of the costal arches by 
digitations which correspond very closely in number with those ribs that do not reach 
the sternum directly through their costal cartilages—that is to say, the false or vertebro- 
abdominal series of ribs. 
This thin sheet of muscle becomes more and more arched as its slips sink lower on 
the series of ribs. When therefore it contracts, each digitation will either draw its own 
particular rib nearer to the central tendon or else maintain the ventral end of its rib 
at a more or less definite distance from the central tendon. 
In this way the ventral ends of the false ribs are provided with temporary or inter- 
mittent fixed points, fixation by the contraction of the diaphragm heing substituted 
for fixation by the sternum, as is the case with vertebro-sternal ribs. In fact, the series 
of ribs could with effect be classified as vertebro-sternal and vertebro-diaphragmatic. 
The flattening of the arched surfaces of the diaphragm must increase the available 
thoracic space, but under ordinary conditions the addition so provided cannot of itself 
be very great, and only becomes important as the central feature of a larger movement. 
Developmentally, the last part to be added to the diaphragm is also its weakest 
part both in man and in the Weddell seal. This is the dorso-lateral segment, which 
consists of muscular fibres forming a delicate sheet extending between the dorso-lateral 
aspects of the central tendon and the ligamenta arcuata externa and interna, and 
through these with the vertebral column on the one hand and the last rib on the other. 
The arched course of these fibres in man must enable them to aid the flattening of the 
dorsal parts of the diaphragm and thereby again assist in pushing the abdominal con- 
tents in a ventral direction, but in the seal they are so feebly developed that the effect 
of their contraction must be practically negligible. I do not doubt that contraction of 
the diaphragm may produce some depression of the central tendon, more especially at 
its dorsal side, but I doubt whether the depression of the central tendon can take place 
on its sternal side or be so pronounced as a whole as to give the “piston action” 
described by some observers. My reasons for hoiding this view may be shortly 
summarised. ‘The pericardial bag rests by its base upon the diaphragm, and, when a 
central tendon of the trefoil pattern is present, the fibrous bag and the central tendon 
are intimately united to each other, but the ventral surface of the pericardium is 
attached to the manubrium sterni by a sterno-pericardial ligament which is described 
TRANS. ROY. SOC. EDIN., VOL. XLVIII., PART II. (NO. 17). 50 
