560 THE DISEASES AND DISOEDERS OF THE OX. 



the diseases according to either seat or nature, we are met at 

 once by difficulties which prevent any complete arrangement/' 

 Many diseases affect more than one part of the nervous system, 

 while the precise seat of some others is not known. A rough 

 division is often made into two broad classes of " organic" and 

 ** functional " disease. In *' organic diseases " there is a visible 

 lesion. The " functional diseases " include (1) those which 

 consist only in a disturbance of function, and (2) transient 

 diseases which are not known to depend on organic changes. 

 Nevertheless, there is also a large class of diseases in which no 

 constant lesion has been discovered, but which are not transient. 

 The use of the microscope in pathology wonderfully extended 

 the range of organic disease, and quite recently a still further 

 extension of our powers of observing organic change resulted 

 from the use of staining agents. It is, of course, the degree of 

 alteration compared with the degree of our aided powers of 

 vision which determines the question of visibility. Molecular 

 changes in nutrition probably cause some diseases classed as 

 functional. Hence " nutritional diseases " may be distinguished 

 from those " functional diseases," properly so called, which 

 consist merely in a derangement of function. Yet we must 

 remember that, if prolonged, functional disturbance will lead to 

 nutritional change. We may, then, distinguish four classes of 

 nervous diseases. 



1. — Organic disease, such as tumour, hemorrhage, softening. 



The morbid process always begins outside the nerve- 

 elements. 

 2. — Structural disease, such as most forms of sclerosis. The 



morbid process may begin either in the nerve-elements 



or outside them. 

 3. — Nutritional disease, such as general paralysis of the insane, 



paralysis agitans, chorea. The nerve-elements are 



probably always primarily affected. 

 4. — Functional disease, such as reflex convulsions, and many 



forms of hysteria. Nerve-elements primarily affected. 



This classification is not very useful, however, for descriptive 

 purposes. 



Myelitis, or softening of the spinal cord, is not of frequent 

 occurrence in the case of this structure of the ox. Tetanus is a 



