Hawaiian Monk Seal — King and Harrison 
291 
channels (Fig. 5). The major portion of venous 
blood is conveyed by interlobular veins reach- 
ing the surface between renules to enter the 
stellate plexus. Some interlobular veins, how- 
ever, pass towards the hilum of the kidney to 
drain into small channels that extend medially 
to the limbs of the posterior vena cava. 
HEART AND GREAT VESSELS 
The aorta is markedly constricted at a point 
immediately to the left of the origin of the left 
subclavian artery and below the ductus arteri- 
osus. This condition is known as coarctation 
( coarctus — pressed together) and is the result 
Fig. 6. A drawing from the right side of a sagittal 
section through the dilated intrahepatic part of the 
posterior vena cava, the hepatic sinus, and the sphinc- 
ter about the intrathoracic part of the vena cava. The 
hepatic sinus is divided by thin, falciform septa; the 
main openings of the hepatic veins are shown as black 
circular areas. 
of partial obliteration of the dorsal aorta either 
between the 4th and 6th arch ( above the ductus 
arteriosus) or below the 6th arch and the dorsal 
aorta (below the ductus arteriosus). It occurs 
rarely in man: Wood (1956) found coarctation 
of the aorta in 9 out of 900 cases of congenital 
heart disease. It appears to be very rare indeed 
in mammals and has not been reported in any 
animals dying at the Zoological Gardens, Re- 
gent’s Park, London (R. W. Fiennes, personal 
communication). Cordy and Ribelin (1950) 
describe its occurrence in a bullock associated 
with dextraposition of the heart. It occurs, in 
man, more frequently in males (4.5: 1 ) , is most 
often found in young adults and 1 per cent of 
the cases have hereditary links (Wood, 1956). 
The transverse diameter of the monk seal 
aorta at the point of coarctation is 1.0 cm., that 
of the first part of the descending aorta is 1.4 
cm. There does not appear, therefore, to be any 
post-stenotic dilatation of the aorta as is often 
found in man. The ascending aorta and its arch 
are dilated with marked thickening of the wall. 
The most dilated part is 4.5 cm. in diameter; 
the thickened wall is 3.0 mm. thick as opposed 
to the wall of the descending aorta, which is 
1.0 mm. thick. At the point of coarctation the 
wall of the aorta is thickened by fibrous tissue 
to 4.0 mm. cranially and to 3.0 mm. caudally; 
the other parts of the aortic wall are less thick. 
Three aortic valves are present (only two are 
present in 23 per cent of human subjects with 
coarctation, Hamilton and Abbott, 1928). The 
aortic ring appears of normal size (aortic ste- 
nosis is present in 7.5 per cent of cases in man). 
The left ventricular musculature appears hy- 
pertrophied, but otherwise the heart is normal. 
There is no patent foramen ovale and the ductus 
arteriosus is closed (7 per cent of human cases 
show a patent ductus ) . The right and left atria 
appear to be of normal size and have walls that 
do not look hypertrophied. There is no evidence 
of enlargement of vessels that provide collateral 
circulations above and below the constriction 
(internal mammary arteries). No notching of 
the posterior margins of the ribs (which can 
be caused by raised blood pressure in the inter- 
costal vessels) is present. The lack of any such 
findings could well be due to the immaturity of 
the animal. 
