138 THE VOYAGE OF H.M.S. CHALLENGER. 



those above referred to in connection with the wing, appear to me to lie in tlie extra- 

 ordinary development of the extensor muscles of the vertebral column — a development 

 which is correlated to the exceptionally erect attitude maintained by the various 

 members of the group while on terra firma. This erect attitude is assumed by these 

 birds so soon as they leave the water, and is maintained not only when at rest but 

 even during the act of progression. The spinal muscles are therefore developed to an 

 extent unknown in other birds, in all of which, with the exception of the Spheniscidas, 

 the trunk of the body occupies a more or less horizontal position during progression, and 

 the necessity of powerful extensor muscles to act on the vertebral column is correspond- 

 ingly diminished. 



J.V.— ANGEIOLOGY. 

 THE HEAET. 



The heart (PI. XL fig. 1) occupies the middle line of the thorax, its apex being 

 accommodated in a depression between the right and left lobes of the liver. From its 

 base the trunk of the aorta passes off, and from the latter the right and left innominate 

 arteries pass obliquely forwards and outwards, being placed symmetrically on either side of 

 the middle line. To the outer side, and rather above the level of the innominate 

 arteries, the corresponding venae cavse superiores pass backwards to the right auricular 

 cavity. The right and left pulmonary vessels extend forwards from the base of the heart, 

 while the inferior vena cava passes onwards from behind to join the right auricle. The 

 heart, as usual, is enclosed in a special pouch of the general serous membrane which 

 constitutes the pericardium. 



The heart in Euchjptes chri/socome from Tristan d'Acunha measures 2g: inches in 

 length, and 2 inches in breadth at the base. The organ is slightly bent upon itself, so 

 that the apex instead of projecting directly backwards is directed backwards and to the 

 right. The grooves which indicate the separation of the auricles from the ventricles, 

 and of the auricles from one another, are clearly defined externally, but the inter- 

 ventricular grooves are scarcely recognisable. 



On opening the right auricular cavity (PI. XL fig. 2) the walls are seen to be almost 

 smooth and devoid of special muscular bands, except at the orifice of the right superior 

 vena cava, where there is a slight indication of their presence. The orifices of the three 

 vense cavse occupy the usual positions. The valve in connection with these orifices 

 consists of two segments, a right and a left, which come into contact in front of the 

 orifice of the right superior vena cava, and behind that of the inferior vena cava. 

 Consequently, both these orifices are guarded by the segments of the valve in question, 

 one segment lying to the right and the other to the left of each. The left segment 



