The =thickness of the walls of the heart= varies in disease. The auricular walls are invariably thin and flaccid except as above noted with diminution of the auriculo-ventricular orifice. The walls of the right and left ventricles differ in thickness in accordance with the distance to which they have respectively to propel the blood and the propulsive effort demanded. Thus the walls of the _right ventricle_ which is only called upon to propel the blood through the lungs are only about ½ an inch in thickness and are thinnest at their lower part. Those of the _left ventricle_ which have to send the blood to the most distant parts of the body are from 1 to 1½ inches except at the lower part where they form the apex of the heart, and are reduced to a tenuity resembling the walls of the auricles. They are thickest at the median part, and diminish slightly in an upward or downward direction. The bulk of these walls is excessively muscular, the fibres arranged as an elaborate double spiral and connected with a layer of white fibrous tissue placed in the interval between the auricles and ventricles and surrounding the auriculo-ventricular openings and the orifices by which the great arteries take their origin. It is at this point, where the muscular fibres of the ventricles are connected with the white fibrous rings, where rupture of the heart usually takes place. The following measurements may be held to refer to medium sized animals of the different kinds mentioned. ─────────┬──────────────┬───────────────────────────────┬────────────── │ Longitudinal │ │ │ Diameter. │Transverse Diameter at the Base│Circumference. │ Inches. │ of the Ventricles. │ Inches. ─────────┼──────────────┼────────────────┬──────────────┼────────────── │ │Antero-Posterior│ Transverse │ │ │ Inches. │ Inches. │ ─────────┼──────────────┼────────────────┼──────────────┼────────────── Horse │ 10 │ 7½│ 5⅓│ 19½ Ox │ 9⅘│ 6⅘│ 4½│ 17½ Sheep │ 4 │ 3 │ 2 │ 7⅓ Pig │ 4½│ 3⅗│ 2⅓│ 9⅘ Dog │ 3⅘│ 3⅗│ 2 │ 8½ ─────────┴──────────────┴────────────────┴──────────────┴────────────── The =internal capacity of the ventricles= is so modified by the amount of _post mortem_ contraction that it differs widely from the actual capacity during life. The left ventricle of the larger domestic quadrupeds usually admits from 3½ oz. to over 5 oz., while the right ventricle whose walls are so much thinner and more lax will contain double that amount. In the smaller animals about a tenth of these quantities will be admitted.