BLOOD-VASCULAR SYSTEM, ETC., OP AMIURUS CAT US. 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 tei'mi nation. 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 atrium 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 trabecules carnce 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 ostium, atro-ventri- 

 ■nhiTP. 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 siirface of the mus- 

 .•ular 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 



