420 SIMPLE TUMOURS. 



turn is in every way similar in structure to that previously form- 

 ing part of the tooth." He again says — " In dental exostosis 



Fig. 85. 



the amount of new cementum may be very slight, or may be 

 considerable in quantity. The affected fang may be but little 

 enlarged, or it may be increased to twice its natural size. Near 

 about the end of the fang is the most common situation to find 

 the greatest amount of cementum ; but you seldom find an 

 increase on one side of the fang only, unless the opposite side 

 has been exposed by the absorption of the gum, or deprived of 

 its periosteal covering. In some cases nodules of cementum are 

 found in various parts of the fang, even near the neck of the 

 tooth." And he concludes that " dental exostosis is caused by 

 that condition of the periosteum which is called irritation — a 

 state usually induced by pre-existing disease in other dental 

 tissues, and in a great majority of cases by caries — not always, 

 however." — {Dental Physiology and Surgery, by JOHN ToMES, 

 Surgeon-Dentist to the Middlesex Hospital.) 



VII. — CALCAREOUS TUMOURS — PSAMMOMA. 



These consist of a deposit of calcareous salts in various parts of 

 the body — in the testicle, parenchyma of organs, and upon nerves. 



Psammoma often constitutes those tumours found within the 

 folds of the lateral ventricles of the brain of the horse, and is 

 important when sufldciently large to induce symptoms of brain 

 pressure. 



