56 DESCRIPTIONS OF PREPARATIONS. 



tures close behind the point of exit of the pulmonary veins : two posterior inter- 

 mediate sacs lying one on each side in front of the corresponding posterior or 

 abdominal sacs, with apertures at the postero-external angles of the lungs. The 

 walls of the sacs are thin, and lined by a pavement epithelium, the ciliated epi- 

 thelium of the bronchi ending at the spot where they open into the sacs. 



The following additional points should be noted in the heart. A thin muscular 

 Eustachian valve protects the entrance of the vena cava inferior. The right auriculo- 

 ventricular valve consists of two muscular flaps, one long and external, the other 

 short, meeting the first at a slight angle and connected to the ventricular wall at the 

 base of the conus of origin of the pulmonary artery, which is long. The left 

 auriculo-ventricular or mitral valve consists of two membranous flaps with chordae 

 tendineae and musculi papillares as in a Mammal. A transverse section across the 

 ventricles shows a very thick walled left ventricle with a thinner walled right 

 ventricle, the cavity of which is crescentic, embracing the left ventricle. Ornitho- 

 rhynchus alone among Mammals shows a similar section. Three semilunar valves 

 guard the base of the aorta and pulmonary artery. The common carotids run up 

 the demi-canal on the ventral aspect of the cervical vertebrae, hidden by the 

 muscles. The jugulars are connected by an anastomosis across the base of the 

 skull, and there is a remarkable subcutaneous venous plexus in the neck. The 

 connections of the veins of the kidney and of the hind limb are characteristic. The 

 femoral vein coming from the hind limb passes through the kidney between its 

 anterior and middle lobes. Close to the internal border of the kidney it receives on 

 its anterior border the efferent vein of the anterior lobe : on its posterior of the 

 middle and posterior lobes. It then passes on as the common iliac vein, and, 

 joining its fellow, becomes vena cava inferior. Towards the outer border of the 

 kidney the femoral vein gives off an afferent renal vein, which enters and ramifies 

 in the anterior lobe of the kidney ; and a second the renal portal, or hypogastric 

 vein which traverses the middle and posterior lobes of the kidney, gives off 

 branches in its course, receives the sciatic vein, and then issues from the 

 posterior lobe of the kidney and unites with its fellow. Into the point of union 

 falls the caudal vein : from it issues a large coccygeo-mesenteric vein, which 

 receives veins from the cloaca and large intestine, runs along the mesentery of the 

 large intestine, and joins the portal system. Each lobe of the kidney receives a 

 small artery. As pointed out by Jourdain, the calibre of the veins is out of all 

 proportion to that of the arteries. He states that each renal lobule contains a 

 central vein, or rootlet of the efferent vein, and is surrounded by a number of venules 

 derived from the afferent vein. A capillary network connects the two systems of 

 vessels. As there are no valves in the renal portal veins, with rare exceptions, such 

 as certain Ratitae, Bustard, Swan, the blood from the viscera and hind limbs can 

 pass freely either through the iliac veins and thence to the vena cava, or through 

 the coccygeo-mesenteric^ vein to the hepatic portal system. It can hardly be 

 doubted from these facts that the kidney, as in Reptilia, receives venous blood. 



The thyroid is a paired gland, and lies close to the origin of the common 

 carotid arteries. The thymus is also paired, and may be found in young birds as 

 a long gland, one on each side of the neck. The spleen lies on the right side of 

 the proventriculus. The supra-renal capsules are two yellow bodies closely 

 connected to the iliac veins at the anterior end of the kidneys. 





