For these reasons the vesicular murmur is heard at its maximum intensity at this point. Towards the upper and lower limits of this zone are found the great bronchial divisions, so that auscultation should always be practised with the greatest care at this point. The middle zone occupies approximately one-third of the total depth of the thorax. The vesicular murmur becomes weaker as one passes backwards, and finally disappears at a considerable distance from the angle of the hypochondrium, following a curved line the convexity of which is directed forwards, and which is continuous with that limiting the upper zone. These peculiarities are due to the anatomical arrangement and mode of insertion of the diaphragm on the internal surface of the hypochondrium. The inferior zone is very limited, and corresponds externally to the inferior third of the thorax, and topographically to the cardiac lobe or middle lobe of the lung and to the inferior portion of the posterior lobe. As these pulmonary lobes are of comparatively slight thickness, the vesicular murmur is feeble. It can be heard over a trapezoidal space, forming a prolongation of the middle zone, but not below in the region of the sternum or pectoral muscles. The fourth zone extends over the mass of the olecranian muscles. It is of triangular form, in consequence of the inclination of the scapula and humerus. Except in very fat animals the vesicular murmur is readily audible through the muscular mass on the right side better than on the left, on account of the development of the right anterior pulmonary lobe. On the left side the beating of the heart is heard above the pulmonary sounds. Clinically one may hear an exaggeration of the ordinary respiratory murmur whenever the lung is actively exercised, as, _e.g._, immediately after trotting. This exaggeration, however, is often pathological. It is known as “juvenile or supplementary respiration,” when due to the fact that some other portion of the lung is not acting. The respiratory murmur may be lessened in certain morbid conditions, such as emphysema and congestion of the lung, and may completely disappear in pneumonia or broncho-pneumonia, a fact which is even of greater significance. In various pathological conditions the respiratory murmur may also be modified. On the other hand, the movement of air in the bronchi also produces various sounds of importance. A number of different bronchial sounds may be distinguished; these include both inspiratory and expiratory sounds, for sometimes an expiratory sound may become audible and clearly appreciable, or may acquire characters of the greatest importance.