240 SECTIONAL ADDRESSES. 



brought a flock of 1,760 sheep on to a farm where this plant was known 

 to be present, and left them there for less than 24 hours. They were then 

 removed to a clean farm and for about six weeks nothing happened. 

 Thereupon the sheep suddenly started dying and within a few weeks 

 1,047 sheep had died. This strange occurrence is explained as follows : 

 the poison contained in the plant acts on the heart muscle causing a 

 myocarditis with subsequent dilatation of the ventricles. As soon as the 

 process has reached a certain stage the animal dies of ' heart failure.' To 

 the casual observer the disease presents all the characteristics of an infec- 

 tious disease ; in the case quoted above it certainly seemed as if the disease 

 had ' spread ' rapidly among the flock. 



The elucidation of the cause of the disease was of great practical 

 importance inasmuch as it enabled the farmer to enclose that portion of 

 his farm where the poisonous plant grew, and to keep his sheep away 

 from it. 



Other no less remarkable diseases were studied by Theiler and his 

 co-workers. 



A disease called ' Geeldikkop ' (yellow thick head) in sheep was shown 

 by Theiler (1928) to be due to a plant Trihulus terrestris, although more 

 recent work by Quin, Steyn and others at Onderstepoort has shown that 

 there are other factors to be considered in the causation of this disease. 



' Vomiting disease ' of sheep was studied by Du Toit (1928) and proved 

 to be caused by Geigeria spp. The disease may produce very severe losses 

 in certain years, especially after droughts, when the plant is very wide- 

 spread. 



Many other instances could be cited of diseases which assume great 

 economic importance and which have been traced to poisonous plants. 

 One final example may suffice to demonstrate the peculiar behaviour of 

 some plant poisons. A disease of horses known as ' Jaagsiekte ' was 

 proved by Theiler (1918) to be due to Crotalaria dura. When the plant is 

 fed to horses it produces, after a long ' incubation period,' a fever and 

 certain characteristic changes in the lungs. On the other hand, if the 

 same plant is fed to cattle it produces equally definite changes in 

 the liver. 



The study of poisonous plants is now being actively pursued in various 

 countries, and further interesting developments may be expected. It is 

 obvious that the co-operation of botanists is essential for the success of .' 

 this work. 



8. Deficiency Diseases. 



Lack of time prevents me from referring in any detail to this interesting 

 group of diseases. 



The great importance of the vitamins in the nutrition of human beinga 

 is so well known that it need not be stressed here. In the case of the! 

 common domestic animals (except perhaps the pig, the dog and the fowl) 

 the vitamins seem to be of far less importance than in human beings. 



On the other hand, mineral deficiencies are, generally speaking, much 

 more important in animals than in human beings. The reason for this is 

 not far to seek : animals, in most cases, derive their nourishment directly 

 from the products of the soil in a limited area ; and if the soil should be 



