BLOOD-VASCULAR SYSTEM, ETC., OF AMLURUS CATUS. 419 
ventral surfaces of the sinus venosus and ductus Cuvieri, and con- 
tinued over their anterior surfaces. 
The sinus venosus lies between the pericardium and the ‘aponeu- 
rotic wall,’ and is but little larger than the sinus-like vessels of 
which it is the termination. Its anterior surface is attached to the 
posterior surface of the atrium in the median line of the body. The 
opening between them is guarded by a pair of large semi-lunar valves 
which not uncommonly become united at their extremities and pre- 
sent the appearance of a diaphragm with a central opening, the ordi- 
nary slit, 3°5 mm. in length, being reduced to a more or less rounded 
passage as small as 1 mm. in diameter. 
The atriwm is a flattened chamber, 14 mm. long and nearly as 
broad at the posterior end. It lies to the left and over the dorsal 
surface of the ventricle, extending from behind its apex to the anter- 
ior extremity of the bulbus. ‘The thick rounded posterior border of 
the atrium is divided into two lobes; laterally and anteriorly the 
chamber thins out to an edge and narrows anteriorly to a blunt apex. 
The wall is formed of connective tissue and is very thin. To this 
wall the trabecule carne are attached and run in various directions 
along the wall and across the chamber, leaving, however, several free 
spaces. The largest of these spaces is opposite the opening into the 
‘ventricle, and the muscle-bundles which surround it are directed to- 
ward this point and expel the blood by drawing the wall of the 
atrium toward the opening, while by-the same contraction they ex- 
pand it. The wall of the atrium surrounding the ostewm atro-ventri- 
culare is strengthened by a muscular ring and thickening of the con- 
nective tissue. The union of the atrium and ventricle is effected by 
the attachment of the outer surfaces of the connective tissue of each 
wall. Where this takes place the connective tissue sends strong in- 
terlacing processes into the muscular ring and the muscles of the 
ventricle. At places muscular tissue also passes from one to the 
other. Where not interrupted by these muscles the connective tissue 
of the wall joins similar tissue covering the inner surface of the mus- 
cular ring to which the pair of vertical semi-lunar valves closing the 
opening are attached. : 
The ventricle is somewhat cylindrical in form and slightly curved 
towards the dorsal surface. The connective tissue-coat is as thick as 
that of the atrium. The muscular tissue is divided into two distinct 
portions, an outer layer, the muscles of the wall, and within this the 
