212 PHYSIOLOGY OF MUSCLES AND NERVES. 



fibre of some length is irritated at one point, an electric 

 change at first occurs only at this point ; this continues 

 an extremely brief time, and then runs wave-like along 

 the muscle-fibre ; and this electric change is then fol- 

 lowed by the mechanical change of contraction and 

 thickening, which is called contraction, and which then 

 propagates itself in a similar wave-like manner. If, 

 however, the w^hole fibre is irritated at once, the elec- 

 tric change occurs simultaneously throughout the fibre, 

 and this is then followed by the mechanical change. 



5. The glands are in many points very similar to 

 the muscles, though their structure is so different. A 

 gland of the simplest form is a cavity lined with cells, 

 opening by a longer or shorter passage through the 

 outer surface of the mucous membrane, or the outer skin 

 (corium), which lies above it. The cavity maybe hemi- 

 spherical, flask-shaped, or tubular. In the latter case the 

 tube is often very long, and is either wound like a thread, 

 or is coiled, and is sometimes expanded at its closed 

 end in the form of a knob. These are all simjple glands. 

 Compound glands are found when several tubular or 

 knob-shaped glands open with a common mouth. Sub- 

 stances, often of a very peculiar character, are found 

 within the glands, and are secreted on to the outer 

 surface through the mouth. These are the sweat and 

 fat of the skin, which are prepared in the sweat or fat 

 glands of the skin, the saliva and the gastric juice, 

 which, owing to their power of fermentation, play an 

 important part in digestion, the gall, which is formed 

 within the liver, and other substances. The similarity 

 alluded to between the muscles and the glands consists 

 in the dependence of both on the nerves. If a nerve 

 which is connected with a muscle is irritated, the muscle 



