539 



THE KIDNEY 



The vertebrate kidney consists of a large number of coelomo- 

 ducts which join together and have a common opening to the 

 exterior. In mammals each coelomoduct or tubule (p. 615) 

 opens internally, not into the general coelom but into a separate 

 minute part of it, called a Bowman's capsule, into which is pushed 

 a knot of blood vessels called a glomerulus. The capsules are lined 

 by pavement epithelium, the tubules by cubical, and lower down, 

 by columnar. The larger vessels, as we have seen, have transitional 

 epithelium. A low power section of a kidney shows an outer 

 cortex consisting mostly of the capsules and convoluted tubules, 

 and an inner medulla, consisting of the collecting tubules leading 

 to the pelvis of the kidney, where the ureter leaves. All the 

 tubules are held together by areolar tissue. 



MUSCLES AND BONES 



The ways in which striped muscle fibres are joined together 

 to form a muscle, and muscles joined to bones by tendons, have 

 already been described. The contraction of a voluntary muscle 

 generally moves one part of the skeleton on another ; the point 

 where a muscle is attached to a relatively fixed part of the skeleton 

 is called its origin, that where it joins a freely movable part its 

 insertion, but these are somewhat loose and arbitrary terms, as 

 according to the circumstances it may be one or the other bone 

 which moves. To enable movement to take place there must be 

 a joint, or in anatomical language, a diarthrosis. The bony surfaces 

 of the articulation are separated from each other by hyaline 

 cartilages, and these are in contact except for a thin lubricating 

 layer of synovial fluid (Fig. 334). Enclosing the articular cartilages 

 and the fluid is the synovial membrane, a mesothelium of con- 

 nective tissue cells, so that the bones are separated by a bag- 

 like space, the synovial cavity. Synovial fluid is very similar to 

 blood plasma without colloids, but it also contains mucin, the 

 lubricating protein. Besides its function in reducing friction 

 synovial fluid probably also supplies food to the cartilage. In a 

 symphysis, where there is slight movement, the bones are covered 

 by plates of hyaline cartilage, and these are joined by fibrous 

 tissue or fibrocartilage. In sutures the bones are interlocked by 

 their wavy edges, as well as being joined by fibrous tissue, so 



