METABOLISM IN VERTEBRATES 



must be studied by means of the microscope. This particular study of structure 

 is known as histology, or microscopic anatomy, in contrast to gross anatomy, 

 or the study of the organ systems by dissection. If we consider tissues first, 

 we find that they are classified according to structure and function. There 

 are five principal classes of tissue: epithelial, sustentative, vascular, contractile, 

 and nervous. 



Epithelial Tissue. The cells of epithelial tissues are compactly placed 

 with but a small amount of intercellular material. Their functions include 

 the covering and protection of body surfaces, both internal and external, as 

 well as absorption, secretion, and excretion. According to the predominating 

 shape of the cells, this class is subdivided into squamous and columnar 

 epithelium, each of which is again divided into simple or stratified, depending 

 on whether it exists in single or multiple layers. The cells of simple squamous 

 epithelium when viewed from the surface resemble tiling blocks; seen from the 

 edge, they are very thin (Fig. 3.13^). Such epithelium is found lining the 

 coelom; that is, it forms the peritoneum (Fig. 3.135). In stratified squamous 

 epithelium only the outermost layers are typically flattened cells; in the deeper 

 layers the cells are progressively more cuboidal (Fig. 3.13C). Since blood 

 vessels do not penetrate epithelial layers, only the cells of the deeper layers 



Fig. 3.13. Epithelial tissues. A, simple squamous epithelium from the human mouth. B, 

 simple squamous epithelium (mesothelium) from peritoneum. C\ stratified squamous epithelium 

 from the linini^ of the nasal cavity. D, simple columnar epithelium from the mucous membrane 

 of the digestive tract. E, pseudost ratified ciliated columnar epithelium from the lining of 

 the trachea; one cell is shown secreting a drop of mucus. F, simple ciliated columnar epi- 

 thelium. C, glandular epithelium from the pancreas. H, goblet cell with a drop of mucus. 

 A and B, surface views; C //, sections at right angles to surfaces which are toward the top 

 of the page. (A, B, and C from drawings by D. F. Robertson.) 



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