130 MORPHOLOGICAL TYPES 



may flow from the caudal vein is the coccygeo-mesenteric vein, 

 which runs downwards and forwards in the mesentery support- 

 ing the intestine, and joins the (hepatic) portal vein. The 

 latter runs from the intestine and duodenum to the liver, as in 

 all vertebrates. There is one more vessel worthy of mention, 

 and that is the epigastric vein which runs forwards from the 

 mesentery, passes ventral to the liver, and joins the hepatic 

 vein. This epigastric vein represents the anterior region of 

 the anterior abdominal vein of lower forms : the hinder part 

 of this vein is represented in the bird by the coccygeo-mesen- 

 teric vein. 



The chief difference, therefore, between the venous systems 

 of the bird and the lizard, is the direct connexion of the " renal 

 portal veins " with the inferior vena cava in the former. It 

 may also be noted how the great development of the pectoral 

 muscles has brought about a modification of the vascular 

 system, in the form of the well- developed pectoral arteries and 

 veins. These muscles, on which the flight of the bird depends, 

 are the most active in the body. 



Urino-genital Systems. — The kidneys are metanephric, not 

 mesonephric. The kidneys lie in the roof of the coelomic 

 cavity, and each is divided into three lobes. Each kidney is 

 connected by a ureter with the urodseal division of the cloaca. 

 There is no bladder. 



In the male, the testes are connected with the cloaca 

 (urodaeum) by the vasa deferentia, or Wolffian ducts. In the 

 female, the right ovary and right oviduct as a rule disappear ; 

 there is then only one ovary (the left) and one oviduct (also the 

 left) or Mullerian duct in the adult bird. The reason for the 

 suppression of one ovary and duct is presumably that if two 

 eggs were to be laid simultaneously (one by each oviduct), 

 their combined size would block the passage between the two 

 sides of the pelvic girdle. The Wolffian ducts are not present 

 in the female, and the male has no Mullerian ducts. 



Nervous System. — In the brain, the cerebral hemispheres 

 are well developed, and considerably larger than in the lower 

 forms. This increase in size is due to the enlargement of the 

 corpus striatum, and not to the development of the cortex or 



