(1) _The more sensitive animals are to venom, the greater is the quantity of serum necessary in order to prevent their intoxication by a given dose of venom._ (2) _For a given species of animal and a given dose of venom, the longer the delay in applying the remedy, the greater is the quantity of serum that must be injected in order to arrest the poisoning._ It will be understood from what has been already stated, that a man weighing 60 kilogrammes, if bitten by a snake which injects, let us say, what would amount to 20 milligrammes of venom if collected in the dry state (the mean quantity that a _Naja_ is able to inoculate in a single bite), would only require, in order to escape death, to receive the quantity of antivenomous serum sufficient to neutralise the portion of venom in excess of the amount that he could tolerate without dying. Let us suppose, for the sake of example, that the man of 60 kilogrammes can withstand intoxication by 14 milligrammes of _Naja_-venom. It follows that, in the case with which we are dealing, we must inject sufficient serum to neutralise 20-14 (=6) milligrammes of venom; that is to say, the injection of serum being made immediately after the bite, 6 _c.c._, if the serum employed neutralises _in vitro_ 1 milligramme of venom per cubic centimetre. Of course, if the serum is more powerful, less of it will be necessary, while more will be required if the remedy is applied later, or if the quantity of venom inoculated by the snake is supposed to have been greater. For this reason, in practice, but very little serum is usually necessary in order to augment the natural resistance of a man of average weight or of a large animal; it is sufficient in most cases to give an injection of 10 or 20 c.c. in order to cure human beings who have been bitten. The clinical proof of this is, moreover, to be found in the cases, already very numerous, that have been published in the course of the last few years in the scientific journals of all countries. I have gathered together a few of these in the concluding pages of this book, and I would beg the reader to be good enough to refer to them. PART IV. VENOMS IN THE ANIMAL SERIES. CHAPTER XVI. 1.--_INVERTEBRATES._ Besides reptiles, many other animals possess poison-glands and inoculatory organs which they employ, either to defend themselves against their natural enemies, or to capture the living prey upon which they feed. The venoms that they produce are still, for the most part, but little understood. A few of them, however, have excited the curiosity of physiologists, especially those secreted by certain batrachians, such as the _Toad_, and certain fishes, such as the _Weever_. Some of them exhibit close affinity to snake-venom, and are composed, like the latter, of proteic substances modifiable by heat and precipitable by alcohol; others possess altogether special characters, and resemble alkaloids.