AN INTRODUCTION TO ZOOLOGY 



c 



FIG. 28. Small portion of pancreas. 



a., alveolus ; c.c., central cells of alveolus ; c.t., con- 

 nective tissue ; d., duct wall (cubical epithelium) ; I.d., 

 lumen of duct ; *.g., zymogen granules. 



represent the stalks and the alveoli the grapes, and for this reason it 

 is termed a racemose gland. The glandular cells contain numerous 



zymogen granules, which, 

 when discharged, give rise 

 to the enzymes of the 

 pancreatic juice that are 

 utilised in digestion. 



The liver, the 

 largest gland in the body, 

 is also a complex gland. 

 It arises as a compound 

 tubular gland, but during 

 its growth the various tubes 

 unite with one another into 

 a close reticulation, so that 

 in the adult its structure is 

 difficult to make out. In 

 section it is seen to consist 

 of large numbers of poly- 

 hedral cells with distinct 

 nuclei embedded in granu- 

 lar cytoplasm. It has also 



a very plentiful supply of blood-vessels. Here and there ducts of 

 different sizes will be seen, and masses of pigment are irregularly 

 distributed in it. The functions of the liver are three in number. 

 In the first place it secretes a fluid, the bile, that takes part in the 

 digestive processes. This bile is collected by numerous small 

 ducts, and thence taken to the cystic ducts. The second function 

 is the storage in its cells of a substance, glyeogen or animal starch. 

 The various sugars obtained from the food are brought to the liver 

 by the blood, and there transformed into glyeogen, which is in- 

 soluble. When sugar is again required by the blood the glyeogen 

 is re-converted into soluble sugar and given up to the blood. By 

 this means the proportion of sugar in the blood is kept fairly 

 constant, and the liver acts as a storehouse in which the excess 

 may be kept until required. Sugar and substances of a similar 

 chemical nature, the carbohydrates, play an important role in 

 supplying the energy necessary for the bodily activities. The 

 third function of the liver is connected with the removal of waste 

 matter from the system. Certain of the waste products are 

 brought to the liver by the blood, and there transformed into a 

 substance called urea. This is secreted back again into the blood, 

 whence it is eliminated by the kidneys. 



