THE ANATOMY OF THE WEDDELL SEAL. 79 



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 frequentlv eneased in an <>s>in'ed 

 shell, the restraining nature of the connection between the first pair of ribs and the 

 sternum is very well marked ; but even in the \Wddell 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 segments. 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 digitatimi 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 being 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 vertebra-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 bet ween the dorso-lateral 

 aspects of the central tendon and the ligamenta arcuata externa and interim, 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 holding this view may be shortly 

 summarised. The pericardia! bag rests bv its base upon the diaphragm, and, when a 

 central tendon of the trefoil pattern is present, the fibruu> bag and the central tendon 

 are intimately united to each other, but the ventral surface of the pericardium i- 

 attached to the manubrium sterni by a sterno-pericardial ligament which is described 



(ROY. soc. EDIN. TRANS., VOL. XLVIII., 329.) 



