What is believed to be the first known case of heart-block arising in a dog as a result of an ingenerate pathological lesion is here reported.

The auriculo-ventricular dissociation was of that degree known as relatively complete block and became apparent on section of the right vagus nerve.

Stimulation of the peripheral end of the cut vagus failed to inhibit the ventricles, although complete inhibition of the auricles occurred. The same results were obtained during the ventricular acceleration produced by strophanthin, so that the failure of the vagus to inhibit the ventricles is not due to the latter's infrequent action, but more probably to a normal lack of direct chronotropic influence upon the ventricular muscle. These findings are similar to those obtained by Erlanger in experimental heart-block.

To small repeated doses of strophanthin injected intravenously the heart reacted as follows: (a) irregular slowing of the auricles and conversion of the relatively complete into an absolutely complete a-v block; (b) a rise in the irritability of the cardiac muscle manifested by a rapidly progressing auricular and ventricular frequency, the ventricular frequency surpassing ultimately the auricular frequency; (c) complete arrest of the auricles, the ventricles continuing at their high rate; (d) sudden fibrillation of the ventricles and shortly afterwards arrest in diastole.

There were found post mortem myxomatous-like thickenings at the free edge of the septal tricuspid leaflet and at the attached margin of the posterior aortic leaflet and along part of the right anterior aortic leaflet. There was also a grayish patch on the right side of the auricular septum above the auriculo-ventricular junction.

The thickenings at the edge of the valves consisted of dense, circumscribed masses of what appeared to be new connective tissue. The same tissue was found pressing against the bundle along the greater part of the latter's course. There was considerable fatty infiltration of the auricular musculature immediately above the bundle and, to a slight extent, of the bundle itself. The fibers of communication between the auricular muscle and the node of Tawara were relatively few as compared with those of the normal heart.

A review of the pathology of heart-block is appended, showing the present status of the question, concerning the relationship existing between disease of the a-v conducting system and the various grades of heart-block.

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