93 



LESSON XVII. 



GLANDS THE SALIVARY GLANDS AND INSALIVATION. 



A gland is an organ which has the power of separating certain 

 materials from the blood which flows through it. Glands are of 

 two kinds ; secreting and excreting. 



A secreting gland (Lat. se, aside ; and cerno, I separate) is one 

 which prepares a substance from the materials of the blood ; in 

 other words, it manufactures a material which did not pre-exist in 

 the blood; this substance being set apart for some future use in the 

 body. Each secreting gland is provided with a system of blood- 

 vesse/s, and generally a duct 



or tube which conveys away i 2 s ._ 



the material prepared. 



An excreting gland (Lat. 

 ex, out ; and cerno) does 

 not prepare a substance, 

 but simply separates a ma- 

 terial from the blood which 

 is afterwards to be expelled 

 from the body. These glands 

 also are provided with ducts 

 for the removal of the ex- 

 cretion the substance ex- 

 creted. The sweat glands 

 of the skin are an example 

 of this kind of gland. 



Fig. 91. Simple Secreting Glands. 



i, straight tube ; 2, sac ; 3, coiled tube. 



Fig. 92. Simple Glands, with pouched 

 sides. 



There are other bodies known as ductless glands, which, as 

 their name implies, have no ducts. The functions of these organs 

 are not well understood, but they generally appear to be concerned 

 in the elaboration of the blood. The spleen is one of the ductless 

 glands. 



Many of the membranes which line the cavities of the body and its organs 

 also have the power of secretion, hence they are called secreting membranes. 

 The amount of substance secreted obviously depends on the extent of the 

 eecreting surface. Hence we sometimes find the membrane drawn up into 

 folds in order that its surface may be increased. On the other hand, we often 

 find the secreting membrane dipping down into the substance beneath in such 

 a manner as to form a multitude of minute tubes, each one having an opening 

 at the general surface. This forms the simplest kind of gland, called the 

 simple tubular gland. In many cases the tubular gland is developed into a 

 coil or convolution, thus adding still more to the secreting surface. The 

 tubular gland is also sometimes enlarged till its shape is somewhat globular ; 



