delivered the opinion of the court.
The defendants in error, mentioned herein as claimants, filed a claim before the. Industrial Commission under the Workmen’s Compensation Act for death benefits to which they say they are entitled as dependents of one Eugene W. Wetz, whom they allege came to his death as the result of an accident, arising out of and in the course of his employment by the City and County of Denver, his employer. The employer carried insurance with the state, insurance fund, designated in this opinion as insurer. Reference also will be made to these parties as defendants. The claimants, being unsuccessful before the commission, instituted an action in the. district court to review the findings and award, which court set aside the order of the commission denying death benefits and remanded the. case with directions to enter an award in favor of claimants. Defendants prosecute a writ of error to review that judgment.
Due to the peculiarity of the commission’s findings and to the fact that the issues may only be determined from the testimony, we deem it advisable to set forth such testimony in some detail in order that the issues be clearly presented and the correctness or incorrectness of the judgment of the trial court determined.
The following facts clearly appear from the record and are undisputed: On the 15th of February, 1936, decedent was working for the City and County of Denver in the *163highway department. He reported for work at about 7:30 in the morning. About 9:15 the foreman sent him across the street to start a Fordson tractor of the street cleaning department. It was a cold morning, the temperature being approximately zero or slightly under. On that morning from fifty to fifty-five trucks had been started in the building where the tractor was standing; the building was large and had been kept closed except when the doors were opened to permit the egress of trucks. The gas discharged by the motors was heavy and hung close to the floor. Such discharge from the motors produces some carbon monoxide which is poisonous and a large amount of carbon dioxide, which is not poisonous, but by occupying space in the lungs prevents the entrance of a normal amount of oxygen. The examination of one of the witnesses, who was working in the building, was in part as follows: “Q. What effect did that have on you that particular morning, if any? A. Well, I absorbed a lot of gas and there was a lot of it that morning and it kind of knocked me out, I don’t need a lot of it.’’ Another witness testified that it “knocked him out” when he went into the garage. The foreman who ordered decedent to start the tractor was asked: “What is the first thing you do, that is, if you were sent over to start a Fordson tractor or truck?” He responded as follows: “Well, the first procedure, of course, as we go over there, we take a hot shot battery to help along in case we can’t start it, but we go over it and open up the circuit there, keep the spark retarded, pull the choke and get around and open the throttle a little bit and get around in front and crank it. If we are unable to start it then we take the hot shot battery. We disconnect the low tension and hook on the hot shot battery onto the coil, then we go through the same procedure, of course, of cranking’. ’ ’ After outlining the foregoing as the customary procedure for starting a tractor the foreman was asked if it required considerable effort to crank one of these tractors and answered in the affirmative. Another witness was asked: *164“Is it customary for them to attempt to crank the tractor before they connect these hot shot batteries? That is, for a mechanic to turn it over once or twice to see if it will run before they connect their hot shot?” He answered: “I would say it would be customary, because it takes some time to connect a hot shot, and if a man is in a hurry he is going to start it the quickest way possible.”
After decedent arrived at the tractor one of the men passing by saw him standing with his left hand on the radiator cap in the position in which a man stands to crank the motor. He did not see decedent actually crank it. This was about ten minutes before he was told that Wetz “was knocked out.”
Shortly after decedent had been directed to start the tractor the man who was to take it out found him sitting on the floor by the side of the machine with a hot shot battery between his legs and with his head lying over on his shoulder; being unable to arouse him he called for assistance and Wetz was carried into an office nearby. A doctor was summoned immediately, who, upon arrival, pronounced the man dead.
Carbon monoxide poisoning as a sufficient independent cause of death, and electric shock from the battery and coils as a contributing cause, are conceded by claimants, in view of the medical testimony, to be eliminated from the case. They now place their reliance on overexertion, under the conditions shown to exist, as the proximate cause of dilatation of the heart and consequent death.
The doctor who performed the autopsy was the only one of the several called as witnesses who saw the conditions thereby disclosed. All the others testified hypothetically. He testified that the examination showed a dilatation of the right auricle of the heart which was caused by something’ of a sudden nature and of recent origin as evidenced by no degenerative changes in the liver which are always found where such a condition is of long standing; that he found a foramen ovale or opening *165from the left to the right auricle; that it was covered by a flap on the inside of the left auricle; that such a condition results from a failure of complete closing of a prenatal opening between the two cavities and is found in 25 per cent of all autopsy cases; that the opening was small; that it was surrounded by scar tissue; that there was no evidence of a recent breaking loose of the flap covering the opening; that the flap was on the left auricle side of the opening where the pressure is greater than in the right auricle, thus tending to keep the flap closed; that from his examination the heart muscle grossly appeared to be in good condition; that in his opinion the foramen ovale as he found it was negligible in determining the cause of death; that he did not believe it had anything to do with the death; that it was so well closed that not more than a drop or two of blood ever got through. This doctor further testified that overexertion could cause the dilatation; that the probability was in favor of the death being caused by overexertion; that the atmospheric condition could have something to do with it and could alone cause it; that carbon dioxide (carbon monoxide being ruled out as the cause of death by a blood test) present in the air would make it harder to get oxygen and would have a tendency to weaken the heart to some extent; that the breathing of air filled with carbon dioxide and a small amount of carbon monoxide are factors that can be contributory to dilatation of the heart; that sometimes a foramen ovale causes dilatation but not one such as this; that dilatation does not occur without overexertion; that there was no condition intrinsic in the heart that would cause death. The foregoing was the testimony of the autopsy surgeon based upon his actual examination and upon deceased’s condition as disclosed by the autopsy.
The lay testimony, including that of the widow of deceased, was to the effect that Wetz’ health had been good and that he never had complained of any trouble.
*166Dr. Buck made only an outward examination of the body. His entire testimony with respect to the cause of death was to the effect that 99 per cent of sudden deaths are due to heart trouble and that as to this particular case he could not tell the cause of death from an inspection of the body but would want an autopsy.
Dr. Blanchard’s testimony showed merely the delivery by him of a sample of deceased’s blood to Dr. Freshman for examination, and the latter’s testimony was to the effect that death was not caused by carbon monoxide poisoning.
Dr. Yegge testified that a heart dilatation might develop: from a foramen ovale, because under some conditions blood might go through the opening even with a flap over it, if the flap were not adherent, and that he did not believe this one was. Asked as to whether the man died from overexertion he answered: “From the testimony this morning I do not believe that I could say whether it was natural causes or overexertion.”
Dr. Burnett testified in effect that patent (unclosed) ovale is a fairly common abnormality and that an enlargement of the right auricle associated with it means a strain on the right side of the heart; that he did not know whether the condition disclosed here in and of itself would cause death, but in view of deceased’s previous healthy condition and color, it could have done so but probably did not; that an undue strain for that individual would be required to cause death; that a heart in the condition this was found to be would not stand as much as a normal heart and might break down under overexertion; that there was a strain on the right side of the heart or there would have been no dilatation; that it was not his experience that death often occurs as the result of acute dilatation without any evidence of trauma or external violence in people who apparently were previously well; that he had known of patients dying in bed from acute dilatation but they were not apparently well previously; that persons may be suffering from *167heart disease and not he aware of it themselves and their condition not apparent to laymen; that in his opinion persons who die of acute dilatation with no history of prior attacks or ailments would disclose on examination something other than simple dilatation such as occurred in this case; that an enlargement of the right auricle is to he expected from the presence of a patent foramen ovale if there is sufficient patency and an overload; that it might develop gradually over a period of years; that if it developed gradually the patient usually would he aware of it, hut might not he, and that he probably would not be in perfect health throughout the period; that individuals with patent foramen ovale sometimes die suddenly.
The testimony of Dr. Dyde was to the following effect: That the autopsy report did not disclose an adequate cause of death; that is, that the dilatation of the right auricle and the patent foramen ovale did not seem to be a sufficient and adequate cause of death; that dilatation may come suddenly in certain diseases or may develop over a long period of time from strain; that a dilated auricle with no organic disease of any kind would be a congenital defect which would not be brought on by overwork; that he could not say what did cause this death; and when asked if it could be caused from overexertion his answer was, “like anybody else,” but that he could not surmise.
It will be observed that the foregoing medical testimony tends in no way to negative the fact that if exertion were present under the atmospheric conditions obtaining it would be a contributing’ cause to the dilatation disclosed by the autopsy report.
We think the foregoing presents a situation in which there are circumstances disclosed by the evidence sufficient to prove that the deceased shortly before his death had engaged in cranking the tractor, and there is direct testimony that the motor was cold and that cranking it *168in such condition requires considerable exertion. Tbe dilatation, under the medical testimony, is adequately explained as resulting from exertion in the existing atmospheric condition concerning which there is no dispute. There was testimony that the dilatation might result from a patent (unclosed) foramen ovale, but there is no testimony that it would result from one that is closed and the positive testimony of the only doctor who saw the condition is that it was closed by a flap on the side of the greater ¡pressure which would tend to keep it closed. All the circumstances were consistent with the fact of deceased having exerted himself to the extent required to crank the tractor. The dilatation, which is not controverted, under competent medical testimony is adequately explained by overexertion under the atmospheric condition shown to be present and, in the opinion of the autopsy surgeon, by the atmospheric condition alone. That these conditions, if existent, do adequately account for it was not contradicted by any medical testimony.
The claimants were not required to demonstrate the cause of the dilatation, but merely to show its cause by competent evidence. Circumstantial evidence is competent. Even in a criminal case, circumstantial evidence is sufficient to convict, if the jury is convinced by it of defendant’s guilt and find the circumstances consistent with guilt and inconsistent with any reasonable hypothesis of innocence. Having introduced competent evidence to prove exertion shortly before death and having shown as a circumstance a dilatation — -adequately explained by exertion — the commission, in the discharge of its function under the law, should have considered the evidence and should have made a finding as to the fact that the evidence was competent to prove; namely, as to whether there was exertion. Instead, the commission found “neither is- there any evidence that decedent at any time exerted himself strenuously or at all.” This *169is a eonelnsion of law. As pointed ont in United States F. & G. Co. v. Industrial Commission, 96 Colo. 571, 45 P. (2d) 895, “Wliat constitutes evidence is a question of law.” An erroneous finding- that there is no evidence of exertion, when the record discloses such evidence; is not equivalent to a finding- that there was no exertion. The existence or nonexistence of exertion are the relevant facts. The duty of finding- one or the other of such facts, when an issue under the evidence, is mandatory on the commission.
In the lig-ht of the uncontroverted circumstances that deceased had always appeared to laymen to be in good health and that he had never complained of ill health; that he was doing work that customarily involved considerable exertion; that he was breathing an atmosphere charged with a small amount of poisonous carbon monoxide and a large amount of carbon dioxide sufficient to affect noticeably two other workmen; that in the light of the uncontroverted testimony of the autopsy surgeon either exertion or the atmospheric condition alone could cause dilatation; in the light also of the uncontroverted fact that dilatation did occur; and in the absence of any testimony of other medical experts as to any other probable causes of dilatation where there is a foramen ovale closed by a flap as deceased’s was, we think the trial court was right in holding- that as a matter of law there was uncontroverted evidence of a sufficient cause of the dilatation; namely, either overexertion or a gas laden atmospheric condition, or both. A determination that the evidence is competent and that it is uncontroverted is the determination of questions of law. That it necessarily follows from a determination of propositions of law that on uncontroverted evidence as it stands the commission must find the death was caused by accident, is not an invasion of its fact finding function. This was pointed out by our court in Skaggs Co. v. Nixon, 97 Colo. 314, 50 P. (2d) 55. In that case we used the following language that in principle is applicable to the present case: *170“We hold, therefore, that the district court was correct in determining that there was competent evidence supporting the claim that Nixon was an employee of the Skaggs Company and that such evidence was uncontroverted. When the court determined these two matters of law, it necessarily followed that the commission, having found that there was no employment when the uncontroverted evidence showed employment, acted in excess of its powers; consequently such finding of fact could not stand. It is contended that the court substituted its finding of fact, that Nixon was an employee of the Skaggs Company, for the finding of the commission, but when the court’s ruling is analyzed, the contention appears to be without merit. The commission found the existence of a negative condition — nonemployment. When the court found that there was competent evidence of employment and that it was uncontroverted, it was passing upon questions of law, and not making a finding of fact. The fact of employment followed from the findings of law, but in making findings of law from which conclusions of fact must of necessity follow, the trial court does not thereby usurp the fact finding function of the commission.” The opinion in the case of Carroll v. Industrial Commission, 69 Colo. 473, 195 Pac. 1097, while it does not fully set forth the reasoning employed in arriving at the conclusion there reached, does not, we believe, lay down any other or different proposition from that stated in the Nixon case.
The direction of the trial court in this case was correct. Having found as a matter of law that there was. uncontroverted evidence showing exertion and that exertion, under the circumstances, was an adequate cause of heart dilatation which produced death, and the findings of the commission on uncontroverted evidence clearly showing that such cause arose out of and in the course of the employment, under section 4481, C. L. 1921, it was proper for the district court to order the commission to enter the proper award. Since under the trial court’s *171determination of questions of law it is inescapable that finding’s of accidental death must be made from uncontroverted evidence the court was acting within its powers and within the letter and spirit of the above section in ordering* an award. The making’ of the finding's of fact that necessarily follow from conclusions of law is but incidental to the making of the award under the situation here disclosed.
The judgment of the trial court is affirmed.
Mr. Justice Bouck and Mr. Justice Holland dissent.