THE SKIN AS AN ORGAN OF SENSATION 91 



anterior root and certain muscles may be made to contract. 

 The brain must be stimulated before there can be percep- 

 tion of a nervous impulse. Through the nerve fibers 

 which run from cells of the gray column into and up the 

 white columns of the spinal cord, the nervous impression 

 must be carried on till it affects the special nerve cells in 

 the particular portion of the brain set apart for receiving 

 the particular kind, of nervous stimulus from the particu- 

 lar part of the body. Then sensation results, and through 

 the influence of the will, carried along the efferent motor 

 fibers, motion may be produced in the voluntary muscles, 

 and a great variety of acts may take place in consequence 

 of the stimulation of the sensory organs in the skin. 



DEMONSTRATIONS AND EXPERIMENTS 



28. Simple Epithelium. In connection with the subject of the 

 skin, the general structure of epithelium may be profitably illus- 

 trated. Where frogs are kept in captivity, excellent examples of 

 simple epithelium can be obtained from the moulted skins, for 

 examination with the microscope. 



29. Ciliated Epithelium. If the roof of a living frog's mouth be 

 scraped with a scalpel and the debris thus obtained be mounted in 

 normal salt solution on a slide, there can be found, on examination 

 with the microscope, many cells which show cilia moving actively. 



30. Squamous Epithelium. Scrape the inside of the cheek with a 

 scalpel, mount the debris in water on a slide, and examine with the 

 microscope. Irregular flattened plates can be distinguished, singly 

 or in groups. In most of them the position of the nucleus can be 

 discerned. 



31 . Epithelium in Section. If possible, some prepared microscopi- 

 cal sections of various kinds of epithelium should be studied by 

 the class. Sections of the wall of the digestive tract, of the trachea, 

 of blood vessels, etc., furnish excellent examples. 



32. Section of the Skin. Specially prepared sections of human skin 

 can be purchased. Usually such sections contain hair follicles, sweat 

 glands, and oil glands. 



