CHAP, viii.] ORGANS OF RESPIRATION AND SECRETION. 233 



vertebral column, a little behind the attachment of the diaphragm 

 (Fig. 105). They lie against the dorsal wall of the abdominal 

 cavity above (i.e., the dorsal side of) its peritoneal lining. The 

 anterior end of the right kidney adjoins the posterior surface of the 

 liver. The left kidney is in proximity to the spleen. 



Each kidney is a rounded body, smooth externally, and showing 

 superficially an arborescent network of veins (Fig. 109, A.) It is 

 convex everywhere, except on its inner side, where it presents a 

 marked concavity. From the middle of this concavity a tube, called 

 the ureter, proceeds inwards and backwards to the bladder (Fig. 105,). 

 This tube is the duct of the renal gland, and it emerges from a 

 fissure in the concave surface, called the hilus of the kidney. From 

 this hilus the renal vein (Fig. 109, A, v) also issues, and into it the 

 renal artery (a) and the nerves enter. The artery enters on the 

 dorsal side of the emergence of the vein. 



As to its structure, the kidney consists of an immense multitude 

 of minute tubes, with vessels, nerves, connective tissue and fat, all 

 enclosed in a thin but firm fibrous coat, which closely invests the 

 gland and contains elastic fibres. 



On making a longitudinal section of the kidney it is seen to be of 

 a more or less red colour and to contain a heart-shaped cavity 

 (Fig. 109, B, pi) towards its inner border. Its solid substance 

 appears divisible into an external layer of a lighter tint, containing 

 minute dark red spots (o) the cortical substance while the rest 

 is darker coloured and forms what is termed the medullary 

 substance. 



The cavity above referred to is the continuation inwards of that of 

 the duct of the gland (the ureter), and is called the pelvis (pi). As 

 the pelvis penetrates the gland it enlarges (i) and surrounds a 

 central prominence which projects into it. The part surrounding 

 this prominence is called the calix. 



The cortical substance forms one continuous layer, and the 

 medullary substance is also arranged in a conical mass or pyramid, 

 the apex of which is directed inwards and projects into the calix as 

 the mammilla or papilla (p), the tubes of which it is composed (t) 

 converging to the mammilla. 



The pelvis is lined with mucous membrane, which is reflected 

 over the apex of the mammillar 



The minute tubes (t) of which the kidney is mainly composed are 

 called TUBULI UKINIFERI. They are very closely packed, and 

 consist of a transparent membrane lined with a polygonal or a 

 spheroidal and glandular epithelium occupying two-thirds of their 

 diameter. These tubuli open on the surface of the mammilla, 

 whence they pass into the substance of the kidney, dividing and 

 subdividing, but continuing a nearly straight course till they come 

 to the cortical layer, where they become much smaller in size and 

 variously contorted in all directions, whilst they freely anastomose. 

 Scattered about in the cortical substance are small capsules (the 

 red specks already spoken of) or Malpighian corpuscles. These are 



