But notwithstanding the value of Mr. Brodie's experiments, there are two circumstances which seem to have been overlooked or disregarded by him, and which it will be necessary to examine, before we can draw the conclusion which, at first view, appeared so naturally to flow from them. In the first place we must inquire into the degree of cooling effect which is caused by the inflation of the cold air into the lungs, and compare this with the opposite effect which might be produced by the generation of the carbonic acid. In the natural or entire state of the animal, the respiration is regulated partly by the influence of volition, and partly of instinct, the quantity of air inspired being just sufficient to an

circulation and nervous energy, but probably little on respira tion," and in the list of authors that he subjoins, he designates Mr. Brodie's paper in the Phil. Trans. as the most important treatise on the subject; Med. Lit. p. 108. His opinion, however, upon this point seems to be somewhat vacillating, as in a subsequent passage, p. 503, he supposes that the chief use of the red particles of the blood, and "of the process which is carried on in the lungs, is to preserve the temperature of the body," and remarks, that the "process may be explained according to the ingenious and important theory of Dr. Crawford;" but adds, "that the nervous system appears to have an influence over the process, without which it cannot be carried on," p. 504, 5. Dr. Thomson also conceives that Mr. Brodie's experiments have entirely destroyed the foundation of Dr. Crawford's theory; Chem. v. iv. p. 632. Mr. Earle, who has brought forwards some, very interesting pathological facts in support of the connexion between the nervous action and the evolution of animal heat, seems to consider the chemical theory as overthrown by Mr. Brodie's experiments; Med. Chir. Tr. v. vii. p. 173, et seq.

expe

swer the demands of the system; but in the riment which has been related, the air is forcibly sent into the pulmonary vesicles, without any cooperation from the animal, and, of course, independently of the correspondence between the different functions which are connected with the action of the lungs. Now, as we have already observed, respiration may be considered as a compound process, one essential effect of which is to abstract heat from the body; the question therefore to examine is, whether the heating or cooling effect of respiration will preponderate, under the circumstances in which the animal was placed in these experiments. The second point in which Mr. Brodie's conclusion is premature, respects the part of the circulation in which the heat is extricated. If according to the theory of Crawford, it is not in the lungs, but in the capillary vessels, that this operation is carried on, we should scarcely expect to find the temperature of the animal supported by the artificial inflation, because the immediate effect in this case is to render heat latent, which is necessarily a cooling process, while it is by no means surprising that the disengagement of the latent heat, which takes place in the natural situation of the animal, should have been entirely prevented, or very much diminished, where the functions of the system generally, and those of secretion and assimilation in particular, were so much deranged, or even entirely suspended. Perhaps, therefore, it

2 See the judicious observations of Dr. Philip; Inquiry, p. 205.

may not be going too far to assert, that in the case in question, where a quantity of cold air is forcibly propelled into the lungs, and a portion of heat necessarily rendered latent, by the conversion of venous into arterial blood, and where we may also presume that the air which is discharged from the lungs, would carry off a quantity of aqueous vapour, and in this way produce a farther abstraction of heat, while, on the other hand, it is not improbable, that the various processes by which the blood is venalized in the natural state of the system, must be impeded or deranged, the result which we ought to expect would be the cooling of the blood, as Mr. Brodie found it to be in his experiments.

These theoretical considerations might have induced us to pause before we admitted the conclusion, that the chemical change of the air in the lungs is not the source of animal heat; but we have two sets of experiments, one by Dr. Philip, and a second by Legallois, which directly oppose those of Mr. Brodie, by showing, that if the attending circumstances are duly considered, we can clearly trace a connexion between the diminution in the quantity of oxygen consumed and the deficiency in the evolution of heat, when we employ the process of artificial inflation. Dr. Philip conceived that the cause why the tempe rature of the body diminished more rapidly where this operation was employed, than where the animal was left undisturbed, depended upon too large a quantity of air having been propelled into the lungs, and he accordingly found that when a less quantity

power

is

of air was used, the cooling process was sensibly retarded by the inflation, so as directly to obviate Mr. Brodie's objections, and to exemplify the of the lungs in generating heat. That this power not directly dependent upon the nervous system appears also to be proved, by an experiment in which the rate of cooling in a dead animal, where the brain and nervous system had been removed, was compared with one which was left in its entire state, in which ease, and also when the artificial respiration was employed, no difference could be observed in the temperature of the animals.+

A very elaborate and ingenious train of experiments was performed upon the same subject by Legallois, the results of which were decidedly favourable to the chemical theory, and fundamentally coincided with those of Dr. Philip. The experiments

3 Inquiry, c. 10. p. 197, et seq. The precise subjects of inquiry which Dr. Philip proposed in these experiments were ; 1. Whether the nervous power is capable of evolving heat from the blood, after the sensorial power has been destroyed; and 2. Whether under these circumstances, more heat is evolved by artificially supporting the circulation, than by leaving the animal undisturbed. The animals employed in the experiments were rabbits, and they were killed by a blow on the occiput; a comparison was made both between the effect of inflating the lungs with more or less rapidity, and inflating the lungs and leaving the animal undisturbed, see ex. 64, 5, 6. We are informed that in one experiment the temperature was actually raised by nearly 1°; p. 199, 0. Dr. Hastings performed similar experiments with the same results, p. 200. See also Quart. Journ. v. xiv. p. 96, 7.

4 Inquiry, p 101, 2. Ex. 67, 8, 9.

consisted in observing the rate of cooling in animals under different circumstances, and in comparing the effect of the inflation of the lungs upon perfect and upon mutilated animals, and also in noticing the degree in which animal temperature is influenced by various impediments to the respiration or to the action of the other functions.5 He deduced the following important conclusions from his experiments, 1. That the animals upon which artificial respiration has been employed, although they suffered a reduction of temperature, yet it was less by from 1° to 3o (cent.) than in simply dead animals; 2. That in cooling through a certain number of degrees, they parted with more heat than simply dead animals ; 3. That inflation of the lungs of perfect and healthy animals lowers their temperature, and that if the operation be continued for a sufficient length of time, they may even be destroyed by cold; and 4. That the same effect may be produced by any circumstance which constrains or impedes the respiration. One important point still remained to be decided; when the cooling process is going forwards by any constraint or impediment to the respiration, is the consumption of oxygen proportionably diminished? Many causes conspire to render the investigation one

5 Ann. de Chim. et Phys. t. iv. p. 5, 113. In Legallois' work "Sur la Vie," he gives an account of the effect of artificial respiration, which he found to have the power of reducing the temperature, according to the observation of Mr. Brodie, although, as he conceived, not in so great a degree; Avantpropos, p. xx; also note in p. 241, 2.