14 



occur during the progress of those diseases, and are charac- 

 terized by the moderate degree of vascular excitement 

 attending them. Hereditary predisposition is the chief 

 cause of tuberculosis, but other causes also promote the 

 diathesis, such as change from a warm to a damp locality, 

 want of proper food, light and warmth, dark, foul stables, &:c. 

 Tuberculosis may be defined (pathologically) as a con- 

 stitutional tendency to the formation of blood, the plasma 

 of which is defective in organizable capacity, so that instead 

 of healthy tissue, it forms abortive blastema, which accumu- 

 lates as a deposit, which is called tubercle. 

 The effects of this deposit are : 



Obstruction and arrest, or impairment of functions. 

 Inflammation, as in tubercular pneumonia. 

 Ulcerative destruction by the repeated formation and 

 softening of the matter, which produces cavities in 

 the organs involved. 

 The organs most frequently attacked are the lungs and 

 the lymph glands. 



The order of frequency with which different organs 

 are affected with tubercle is as follows : 



Lungs. Spleen. 



Intestines. Kidneys. 



Lymph glands. Liver. 



Larynx. Bones. 



Serous membranes. Uterus. 



Brain. Testicles. 



The parts especially preferred by tubercle for its deposit 

 are in the lungs, the apex, in the pia mater, about the base 

 of the brain, the grey substance, in the bones, in the bowels, 

 the lowest part of the ilium, &c. 



Hypertrophy is strictly an over-growth, an increase of 

 the size and weight of a part without change of tissue. It 

 is only in recent times that this has been clearly distin- 

 guished from enlargement with alteration of tissue. 



A constant law of the animal economy is, that within cer- 

 tain limits the growth of an organ is in proportion to its 

 exercise, provided this exercise is not too violent, and is 

 alternated with sufficient periods of repose. 

 The three causes of hypertrophy are : 



1. Increased exercise of a part in its healthy functions. 



2. Increased accumulation in the blood of the particular 



materials which a part appropriates in its nutrition 

 or secretion. 



3. Increased efflux of healthy blood to the part. 



