[118] When two states, which are not perfectly similar, are designated by a common name, it is very difficult, whatever care may be taken to distinguish them, not to apply to one something which exclusively belongs to the other. This is perhaps one of the most frequent sources of our errours. In this case, for example, it does not seem that there is a great inconvenience in designating by the word sleep the state of torpor of certain animals during a part of the year. It is well known that we understand by it altogether a different thing from the sleep, which in warmer seasons of the year, comes on periodically every day; yet in consequence of the identity of the name, we are disposed to admit identity of character and to infer from one respecting the other. [119] What is the circulation of an animal which exhibits no trace of vessels? what inferences can be drawn for man from the mode of nutrition of a polypus? what relation can be established between the complex function which presides in the mammalia over the support of the organs, and the kind of imbibition by means of which the zoophyte is developed and preserved? [120] The external ciliary nerves only come from a ganglion. The internal ciliary ones which have precisely the same distribution and serve also very probably the same uses, come from a cerebral nerve, from the nasal branch of the ophthalmic. [121] The galvanic stimulus usually produces very evident effects upon the contraction of the intestinal tube; these motions are less evident in the stomach than in any other part of the canal; but the same difference is always observed whatever be the stimulus employed. [122] Death does not always take place in the same way. It has been remarked, for example, that those who were hung at Lyons died quicker than those who were hung at Paris. In seeking for the cause of this difference, it was ascertained that in those who were executed at Lyons there was almost always a luxation of the first or the second vertebra, which was owing to a rotatory motion, which the executioner gave to the criminal in throwing him from the scaffold. The death was quick, because it was produced by compression or laceration of the spinal marrow; it was slower in the other case in which it was only the result of asphyxia. [123] It appears by the beautiful experiments of M. Edwards that frogs can live but a very short time in water deprived of air by boiling. Immersed in a small body of water containing air they soon die, no doubt after they have exhausted the air held in solution in the water. They can on the contrary live an indefinite time in this state of immersion, if care be taken to renew the water sufficiently often. The same thing happens, and still more certainly, if they are immersed in running water.