VASCULAR SYSTEM. 441 



tion,, sperm-transference being effected by a slight eversion of 

 the cloaca. In the Ratitae (except Rhea) there is solid grooved 

 penis attached to the ventral wall of the vestibular division of 

 the cloaca ; it is very similar to the corresponding organ of 

 Chelonia, contains erectile tissue and can be extruded or re- 

 tracted by special muscles. In RheM and anserine birds a very 

 similar organ occurs, but its terminal portion is invaginated 

 during rest and evaginated in erection like the finger of a glove. 

 The heart is completely divided into a right and left half, and 

 lies in the median line, enclosed by the pericardium. As a pecu- 

 liarity of the heart may be mentioned, the special development 

 of the right auriculoventricular valve, which, unlike the tri- 

 cuspid valve of the mammalian heart, is a simple strong mus- 

 cular fold. The left auriculoventricular valve is membranous 

 and possesses chordae tendineas as in mammals. There is no 

 diaphragm in the mammalian sense and the thoracic cavity is 

 directly continuous with the abdominal. The pulsations of the 

 heart, in correspondence with the more active respiration, are 

 repeated more rapidly than in Mammalia. The right aortic 

 arch alone persists. The carotids converge and run close to- 

 gether in a furrow on the ventral side of the cervical vertebrae. 

 In some birds they unite, and in some only one of them is pre- 

 sent. In some cases superficially placed vessels {superficial 

 carotids) coexist with or take the place of the normal carotids 

 (deep carotids). There is no sinus venosus. The veins open by 

 two superior and one inferior vena cava into the right auricle. 

 The renal-portal circulation appears to be absent, all the blood 

 from the hinder part of the body passing either directly into the 

 inferior vena cava or by the coccygeo-mesenteric into the hepa- 

 tic-portal system. Th^ coccygeo-mesenteric vein is peculiar to 

 birds ; starting from the point where the caudal vein bifurcates 

 into the two veins (called variously internal iliacs, hypogastric, 

 renal portal) which run forward on the ventral surface of the 

 kidneys to join the femoral veins, it passes in the mesentery of 

 the rectum to join the portal system as one of the roots of the 

 portal vein ; it receives blood from the hinder end of the small 

 intestine, the rectum and the cloaca. There appears to be a 

 representative of the anterior abdominal vein in the form of a 

 vessel which collects blood from the abdominal wall and great 

 omentum and falls into the left hepatic vein ; it does not how- 



