no SCIENCE PRIMERS. [§ VII. 



of a single layer only of cells. The whole of the inside 

 of the canal is therefore red and moist, and whatever 

 lies in the canal is separated by a very thin artition 

 only from the blood in the capillaries, which are found 

 in immense numbers in the walls of the canal. The 

 alimentary canal is, as yci know, a long tube, wide at 

 the stomach but narrow elsewhere. In all parts of its 

 length the tube is made up of mucous membrane on 

 the inside, and on the outside of muscles, differing 

 somewhat from the muscles of the body and of the 

 heart, but having the same power of contracting, and 

 by contracting of squeezing the contents of the tube, 

 just as the muscles of the heart squeeze the blood 

 in its cavities. The muscles, and especially the mucous 

 membrane, are crowded with blood-vessels. 



Though the epithelium of the mucous membrane 

 is very thin, the mucous membrane itself is thick, in 

 some places quite as thick as the skin of the body. 

 This thickness is caused by its being crowded with 

 glands. In the skin the sweat glands are generally 

 some little distance apart, but in the mucous mem- 

 brane of the stomach and of the intestines Ihey are 

 packed so close together, that the membrane seems 

 to be wholly made up of glands. 



These glands vary in shape in different parts. No- 

 where are they exactly like the sweat glands, because 

 none of them are long thin tubes coiled up at the 

 end in a knot, and none of them have a great thick- 

 ness of epidermis to pass through. Most of them are 

 short, rather wide tubes ; some of them are branched 

 at the deep end. They all, however, resemble the 

 sweat glands in being tubes or pouches closed at the 

 bottom but open at top, lined by a single layer of 



