The bark, leaves, and seeds are all poisonous, the leaves usually being the parts eaten. Henslow states that of 32 cases of Yew poisoning 9 were due to the berries and the remainder to the leaves, while 20 of the 32 cases were fatal. In 1879 R. Modlen published a circumstantial account of children poisoned at Oxford by the fruits, and demonstrated that the arillus or scarlet mucilaginous cup surrounding the ripe seed is not poisonous, but that the seeds are so. Apart from idiosyncrasy of appetite stock are perhaps more likely to eat the dark green foliage of the Yew in winter or during scarcity of green fodder than at other times. The old leaves or shoots are the most poisonous parts.
By experiments with autumn and winter leaves Cornevin found that the following quantities would be necessary to kill various animals:—
Horse 0·2 lb. per 100 lb. live weight. Ass and Mule 0·16 lb. „ „ Cow and Sheep 1·0 lb. „ „ Goat 1·2 lb. „ „ Pig 0·3 lb. „ „ Rabbit 2·0 lb. „ „
Other authorities, however, quote much lower quantities as necessary to kill the horse and ox, and Pott says that in the case of horses 150 to 180 grammes (0·33 to 0·4 lb.) is sufficient to cause death in one-quarter to one-half an hour, without previous symptoms. Eaten by an animal on a full stomach a small quantity of Yew may cause little or no dangerous results.
_Toxic Principle._ The researches of Marmé pointed to the alkaloid _Taxine_ (C_{ 37}H_{ 52}NO_{ 10}) as the toxic substance. Later investigations of Thorpe and Stubbs confirmed this view, and it is generally agreed that this substance is the toxic principle, though doubts as to its being the only poison have been expressed. Taxine has a bitter taste, is a heart depressant, and may cause death from suffocation. The yew also contains a considerable quantity of _Formic Acid_, and the acrid, irritant volatile _Oil of Yew_.
Thorpe and Stubbs (_Trans. Chem. Soc._, 1902) found in the fresh leaves of the common Yew from 0·1 to 0·18 per cent. of Taxine. In an investigation of the Irish Yew (_T. baccata_ var. _fastigiata_) Moss found Taxine present as follows:—
Leaves from female tree, fresh 0·596 per cent. Leaves from female tree, fresh 0·623 „ Seeds from fruit of same tree 0·079 „ Another female tree—leaves 0·323 „ Another female tree—seeds 0·082 „