NERVOUS SYSTEM. ' 93 



distension : the carbohydrates are especially apt to give rise to this 

 disturbance. 



What are the requirements of a normal diet ? 



There should be a general diet of well-cooked food, and it should 

 contain about the amount of carbon and nitrogen which is excreted ; 

 that is, it should maintain an equilibrium. This is. commonly, 

 about two pounds of solid food and two quarts of fluid. The pro- 

 portion of the various kinds of foods varies considerably, but in a 

 general way for a healthy man one may divide the solid food some- 

 what in this way : nitrogenous food (meat), about 2 pound ; hydro- 

 carbon and fat-food (bread, vegetables, and butter), about li 

 pounds. Besides this, the food will contain from 1 to 2 ounces 

 of salts and a varying amount of sugar. 



NERVOUS SYSTEM. 



THE NERVES. 

 What are the elementary tissues of the nervous system ? 



Nerve-fibres and nerve-cells. The fibres are of two kinds, and 

 are united in bundles to form nerve-trunks or nerves. The cells 

 are collected in groups and form the nerve-ganglia, but the nerve- 

 fibres are also found in the ganglia. 



What varieties of nerve-fibres occur ? 



1, The medullated or white fibres ; 2, the non-medullated or gray 

 fibres. 



Describe the medullated or white fibres. 



These fibres consist of an external nucleated sheath or p IGi 22. 

 neurilemma which is made up of (1) a layer (Fig. 22) 

 of endothelial cells which surround and invest ; and (2) 

 an inner protective medullary sheath (the white matter 

 of Schwann). Within these is (3) the axis-cylinder, 

 which consists of a number of the primitive fibrillaB 

 of the nervous tissue. In size these fibres vary con- 

 siderably, but the average may be said to be about 



TT01T i nc h i* 1 Diameter. Diagram of 



Structure of 



What are the nodes of Ranvier? 



They are constrictions which occur here and there in the course 



