HORMONES, DRUGS, AND TOXINS 727 



dispersed one, and has, therefore, been stated to be a mixture of "chemically 

 different tannins," although no chemical difference has been shown to exist. 

 Wolfgang Ostwald justly insists that the "chemical" conception is here a purely 

 negative one, and that to be satisfied with the fact that the chemical composition 

 of such substances as "immune bodies" is so complex that one may quietly 

 ascribe all their properties to it, is the antithesis of a view leading to progress. 



The growth of excised tissues in plasma is of interest in this connection. It 

 was thought at one time that such tissues would only grow in plasma of the same 

 individual, an extreme case of specific relationship. But Walton finds (1914) that 

 this is not the determining factor ; any plasma, of the same or another individual, 

 may contain certain substances which inhibit the growth of tissue, together with 

 others which favour growth. The former are destroyed by freezing the plasma 

 for one to three days, the latter by a longer period of freezing, six to eight days. 

 Further, D. and J. G. Thomson (1914) have found that tissue from certain human 

 tumours can be cultivated successfully in the blood-plasma of the fowl, to which 

 has been added extract of embryo chick. Champy et Coca (1914) find the plasma 

 of the cat is toxic for the tissues of the pigeon, while rat tissue grows excellently 

 in tortoise plasma. The toxicity, in fact, is merely accidental. Reference may 

 also be made to the work of Margaret R. Lewis (1915, p. 155) who obtained 

 excellent growth in Locke's Ringer solution. 



We may now proceed to refer, briefly, to certain examples of drugs which have 

 a physiological interest. 



It is remarkable how great a variety of these active substances are formed by 

 plants. It seems evident that they must be more or less accidental products of 

 chemical change. A very small number would suffice for protection of .the plant 

 from being consumed by animals for food. Similar conclusions may be drawn 

 from the occurrence of adrenaline and a substance related to digitalin in the 

 " parotoid " glands of a tropical toad, described by Abel (1911). It is impossible to 

 see what use to a toad a rise of blood pressure in the animal which attacks it 

 could be. 



Acetyl-choline. Dale (1914) finds that this substance produces vascular 

 dilatation in extraordinarily small doses, much smaller than those of adrenaline 

 required to raise the blood pressure. The perfused heart of the frog also shows 

 a distinct inhibition with a dilution of one in a hundred millions. It excites 

 especially the nerve endings of the cranial and sacral autonomic systems, causing 

 stimulation of the vagus, secretion of saliva, contraction of the oesophagus, stomach, 

 and intestine, and of the bladder. The effects, although so powerful, last but a 

 short time. The ester is probably hydrolysed into its relatively inert components. 

 It has scarcely any action on the plain muscle known to be innervated by the 

 sympathetic system. Since it produces vascular dilatation, the non-sympathetic 

 origin of vaso-dilators in general seems to be indicated. 



Strychnine. It is unnecessary here to say more about this alkaloid than to 

 call attention to its peculiar physiological property of converting inhibition into 

 excitation in the phenomena of reciprocal innervation, a property which has 

 already been described (page 427). 



Nicotine. It was shown by Langley and Dickinson (1889) that nicotine has 

 the property, in moderate doses, of paralysing the nerve cells of sympathetic 

 ganglia, without affecting the peripheral endings of the fibres. The effect appears 

 to be exerted on the synapse, and is not confined to sympathetic ganglia. It 

 serves, therefore, as a valuable means of discovering whether there is any cell 

 station for given fibres in any situation to which the drug can be applied. Langley 

 has made considerable use of it for this purpose. The synapses of different nerves 

 require different doses, and also the sensibility to it of different species of animal 

 is not identical. 



Preceding the paralysis, there is a stage of stimulation, so that, amongst other 

 phenomena, a large rise of arterial pressure results from intravenous injection of 

 nicotine. 



Atropine and Pilocarpine. The interest of these two alkaloids, the first in 

 paralysing secretory nerves, the second in stimulating them, has been referred to 



