424 SCHULTE, SEI WHALK. 



origin has here the shape of an inverted V with unequal arms. The longer lateral arm gives 

 rise to the crus laterale, the mesal to the crus intermedium, but only in part for bundles of this 

 portion arise also from the deep surface of the crus mediale. The lateral crus is continuous with 

 the pars costalis and has the same orientation of its bundles. It is separated from the crus 

 intermedium by a fibrous trigone, which cephalad has its apex prolonged into a strand, which 

 receives on its lateral aspect the rostro-mesally directed bundles of the crus laterale, on its mesal 

 side the fasciculi of the intermediate crus, which have a rostro-lateral inclination. On the right- 

 side the strand proceeding from the fibrous triangle joins the centrum tendineum, on the left 

 it is possible to follow it beside the oesophageal orifice, where it turns towards the right as though 

 to join the central tendon by arching between the orificium oesophagi and the foramen quad- 

 rangulare, but here its course is obscured by muscular bundles and it disappears from view. 



The centrum tendineum is large, but is confined to the dorsal plane of the diaphragm. Its 

 most ventral point is the caval aperture which it surrounds with a narrow zone of fibrous tissue, 

 expanding caudad in a wide oval which exceeds on the right the limits of the hepatic adhesion 

 and then tapering is continued by the fibrous strand already described to the trigone of the right 

 side. On the left the margin of the tendon approaches the right mesal crus, but a narrow strip 

 of the crus intermedium intervenes between them. Thus the right leaf of the central tendon 

 is highly developed, the central leaflet is all but entirely replaced by muscle, and the right is 

 represented by a slight expansion in the fibrous interval between the crus intermedium and 

 crus laterale. By means of this structure on the right the central tendon is prolonged to the 

 origin of the pars lumbalis. The structure of the muscle is peculiar chiefly as a consequence 

 of the arrangement of its fibrous tissue. This as a whole may be schematized as an inverted 

 U with its ends at the fibrous trigones and its arch, obscured it is true, ventral to the foramen 

 cesophagi. Into the ectal contour of this U are inserted, converging from the costal arch, the 

 bundles of the pars costalis and caudad also those of the crus laterale. More superficially and 

 partly overlapping the most ventral fasciculi of the costal portion, the sagittally directed pars 

 sternalis inserts upon the arch of the U. The crus laterale, the origin of which extends upon 

 the tendinous arch of the crus mediale, inserts upon the sides of the ental contour of the U, 

 while the crus mediale, itself superficially placed, is inserted into its arch, the right pillar passing 

 ventrad, the left dorsad to the oesophagus and blending at their insertion. As a result of the 

 approximation of the two superficial sets of bundles, the pars sternalis and the crura medialia, 

 the tendinous insertion is concealed from view, and the arch of the U of our schema is buried in 

 the substance of the muscle. 



The phrenic nerves, derived chiefly from the fourth cervical, emerge from the scalenus 

 near its mesal border and passing round this margin descend into the thorax along the venter 

 of the precava. Here the nerve of the left side receives a considerable branch from the fifth 

 cervical. After passing the hila of the lungs the nerves are concealed in the accumulation of 

 subpleural fat about the pericardium. They enter the diaphragm after division into several 

 branches. The inferior phrenic arteries are derived from the aorta caudad of the origin of 

 the superior mesenteric. Before piercing the crus mediale each gives a branch to the adrenal. 

 The phrenic veins empty into the renals. 



The hypaxial musculature. The muscles placed upon the ventral surface of the spine attain 

 an extraordinary and highly specialized development in Balcenoptera. They extend along the 

 whole axis in this position, save for a thoracic interval from the VI to the XIII dorsal vertebra 



