ORGANIC FORM AND ARCHITECTURE 691 



Generate in 1801. The heart is an organ, with its chambers and 

 valves, to be dissected out with scalpel and forceps, but Bich&t 

 penetrated to a deeper level of structural analysis, requiring the 

 microscope, and studied the muscular tissue, the nervous tissue, and 

 the connective tissue involved in the working of the organ in 

 question, for Bich^t was physiologist as much as morphologist. 

 Indeed, in histology [histon, a tissue), it is not very profitable to 

 try to separate the physiological and the anatomical aspects. 



SURVEY OF ANIMAL TISSUES— It is usual to distinguish 



[a) epithelial, (6) nervous, [c] contractile, and [d) connective tissue 

 — the last very much of a lumber-room. Epithelium consists of 

 a layer of relatively simple cells, often united by intercellular 

 cementing substance, and sometimes with protoplasmic bridges 

 running between one cell and its neighbours. In illustration may be 

 mentioned the epidermis covering the outside of the body, the 

 internal lining of the food-canal, the peritoneum investing the 

 interior of the body cavity, the pulmonary epitheUum lining 

 the internal cavity of the lungs. The chief functions of epithehum 

 are protective, absorptive, and glandular. In many cases the cells 

 are columnar, like bricks placed on end, or Hke basaltic columns in 

 miniature; the opposite extreme is seen in squamous epitheUum, 

 where the cells are horizontally flattened, like the slates on a roof. 

 This is well illustrated by the fiat cells which are continually being 

 rubbed off from our lips; while the columnar cells lining our wind- 

 pipe, with lashing cilia on their free surface, illustrate another very 

 common type— ciliated epithelium. The cells of columnar and 

 cubical epithelium usually divide into two in the plane vertical to 

 the free surface, while others, such as those lining the mouth, 

 divide in a horizontal plane parallel to the surface, thus giving rise 

 to what is called stratified epitheUum. What is shed in flakes in a 

 moulting reptile is the outermost layer of the stratum comeum of 

 the epidermis, which gradually dies away; and in snakes this is 

 peeled off, from in front backwards, as a continuous "slough", 

 bearing the imprint of all the scales, which are, of course, left 

 behind. On the other hand, what is cast in a crab or lobster is a 

 cuticle, which may be defined as a non-living and non-cellular 

 renewable product of the underlying living skin. The slough of a 

 snake once consisted of living ceUs, but the moulted cuticle of an 

 Arthropod was never ceUular. 



In many cases the epitheUum is markedly glandular, the cells 

 making some non-living product or secretion at the expense of 

 their living matter. When the epidermis of an ordinary fish, such as 

 a skate, is microscopically exammed, glandular epithelial cells, 

 often shaped like tiny goblets, are seen at frequent intervals among 

 the ordinary covering cells; and it is to their activity that th 



