The microscopic examination of the myocardium showing cloudy swelling gave the usual picture as is seen with a variety of infections. The muscle fibers showed a fine granular deposit in their cytoplasm and the staining quality of the tissue was somewhat altered. The transverse striæ were less distinct than normal, while not uncommonly the longitudinal fibrils became more evident. Fatty degeneration was not encountered. In the single case showing a definite and acute dilatation of the ventricles the cause of the myocardial lesion could not be placed at the door of the influenzal infection. This was the case suffering from a secondary streptococcal bacteriæmia arising in the middle ear. It is more than probable that the streptococcus was the immediate cause of the acute muscle change and weakening. In a number of cases we have studied the tissues of the bundle of His, but we were unable to note any definite change. It is interesting that the intoxication associated with acute influenza is selective in localizing in certain muscle tissues. We have previously indicated the intensity of muscle degenerations occurring in the abdominal recti. Even in these cases where these striped voluntary muscles were markedly affected the myocardium showed nothing more than a mild or moderate grade of cloudy swelling. We can only account for this in a difference in the constitution of these muscular structures, some being of such composition permitting of the localizing and damage by the unknown intoxicant. It does not appear that the reason for localization in certain tissues is in any way related to the character of the blood supply, nor is it related to the activity of the part. In three cases we have found an inflammatory lesion of the endocardial tissues. In all of them this consisted of a slight acute verrucose mitral endocarditis. The lesions were very small, consisting only of a fine granular deposit looking like grains of sand localized along the border of the mitral leaflets. In no instance was the leaflet injured or incapacitated. Unfortunately the lesion not being suspected was encountered after the heart had been removed and opened and when it was too late to make bacteriological analyses. This point is greatly to be regretted, in as much as it is of great importance to know whether some distant lesions are induced through the influenza bacillus or its symbiotic flora. The majority of authors report but little upon the heart lesions in influenza. Many deny that a heart involvement is to be found, a few report an occasional endocarditis. Wallis and Kuskow found more or less myocardial change similar to what is usually described as cloudy swelling. This reaction they point out differs in no way from the degenerations arising from other types of intoxications. Keegan in a series of about 23 autopsies found only a single case with acute dilatation.