GENERAL ZOOLOGY 



Dorsal aorta 



Renal artery 



Glomerulus 



Postcaval vein- 



Renal 

 corpuscle 



Excretory duct 



II Renal portal 



Fig. 3.12. Nephron and blood 

 vessels in the excretory organ of 

 the frog; diagrammatic. The 

 nephron of man is longer and 

 highly convoluted; there is no 

 renal portal vein. 



Inside the kidney, the ureter expands to form the pelvis of the kidney into 

 which many small tubules empty. These carry urine from the very numerous 

 nephrons or excretory tubules to the pelvis. Each nephron ends in a cup known 

 as Bowman's capsule which contains capillaries that arise from branches of 

 the renal artery (Fig. 3.12). Each of these groups of capillaries is a 

 glomerulus, which with its surrounding Bowman's capsule makes up a renal 

 corpuscle. The excretory tubules are lined by a single layer of cells and 

 surrounded by a very conspicuous capillary network. In the frog these capil- 

 laries are fed by the renal portal vein as well as by the renal arteries, but there 

 is no renal portal vein in mammals. 



This completes the very brief survey of the morphology of the organ systems 

 concerned with satisfying the metabolic requirements of the cells of the verte- 

 brate. It is time now to become acquainted with the different kinds of cells 

 found in the vertebrate body. 



The Kinds of Tissues: Histology 



The various systems of the body, or soma, are made up of cells, collectively 

 known as the somatic cells. In Chapter 2 a so-called composite cell was 

 described (p. 17). If the body of a vertebrate is examined microscopically, 

 no cell will be found that conforms to that account; cells differ among them- 

 selves, although they all possess certain features in common. Cells may be 

 dissimilar in shape, position in the body, structure, and also function. Cells 

 that are similar in structure and function make up groups known as tissues; 

 tissues are groups of cells specialized in the same way for the performance of 

 the same function. Tissues are associated to form the organs that perform 

 special functions. The cells as they are grouped to form tissues and organs 



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