'2\'2 MICROSCOPIC ANATOMY OF THE ORGANS. 



No nerve-endings have been found in the walls of the air sacs. 

 Berkley has described arborizations of fine fibrils upon and 

 between the cells of the alveoli. 



IV. URINARY SYSTEM. 

 A. KIDNEYS. 



The kidney is a compound tubular gland ; but it may be 

 considered as an alveolotubular structure, since the urinary 

 tubules are dilated at their ends to form the capsules of Bow- 

 man. There can be distinguished in this organ a medullary 

 and a cortical substance, a marked difference existing between 

 the two parts, in the course and structure of their tubules 

 (Figs. 163 and 164). 



The medullary substance consists of a number of cone- like 

 divisions, the so-called Malpighian pyramids, whose apices 

 extend down into the pelvis of the kidney as papillae. In man 

 the number of these pyramids varies from seven to twenty. In 

 many other mammals there is only a single pyramid and one 

 papilla. These pyramids are made up of straight tubules 

 extending radially from the apex of each papilla to the border 

 of the cortex. From the medulla the straight tubules extend 

 up into the cortex in conical masses, known as the pyramids of 

 Ferrein, or medullary rays. It will be noticed that the Mal- 

 pighian pyramids are many times as large as the pyramids of 

 Ferrein, that their apices point in different directions, and that 

 their bases are approximated. Further, the pyramids of Fer- 

 reiri are situated in the cortical region, while the Malpighian 

 pyramids make up the medulla. 



Each tubule has its origin in the cortex in the region 

 between the medullary rays, in a sac, the capsule of Bowman, 

 into which is pushed a mass of blood capillaries, the glomerulus. 

 The capsule of Bowman with the tubule may be compared 

 with a rubber tube possessing at the end a bulb, the wall of 

 which has been invaginated from the outside by a, body repre- 

 senting the glomerulus. The space in the invaginated sac is 

 the beginning of the lumen of the urinary .tubule. The por- 

 tion of the tubule next to the Bowman's capsule is known as the 



