Direct auscultation is the most certain method, but the ear cannot be applied with equal facility at all points. Under such circumstances a simple or binaural stethoscope, or the phonendoscope, may be used with advantage.
To properly appreciate the sounds heard it is essential to be exactly acquainted with the relation between the lung and thorax. On the left side (Fig. 60) the anterior pulmonary lobe occupies the space between the first and fourth ribs, in front and above the base of the heart. The middle, or cardiac, lobe covers the left upper and postero-lateral part of the heart from the fourth to the sixth rib. The posterior lobe occupies all the region beyond the sixth rib as far as the twelfth.
On the right side the arrangement is similar, but the anterior lobe and the cardiac lobe are more developed (Fig. 62).
Under ordinary circumstances the extensive movement of the lung which occurs during inspiration produces a special sound known as the respiratory or vesicular murmur. Contrary to what has been written, and said, this sound in animals possessing absolutely sound lungs ends with inspiration. Expiration is silent, though it is easy to estimate its duration.
In auscultating the lung, we may distinguish four zones, a superior zone, a middle zone, an inferior zone, and a scapular zone.
The superior zone is bounded by the vertebro-costal gutter, descends approximately as far as the inferior line of insertion of the common intercostal muscle, and extends from the summit of the scapula in front to the hypochondrium behind.
Auscultation of this region through the ileo-spinal and common intercostal muscle will always reveal, except in very fat animals, the vesicular murmur to a point as far back as the eleventh intercostal space. Nevertheless, this vesicular murmur is relatively feeble, and becomes imperceptible beyond the eleventh rib.
The middle zone comprises the most convex portion of the ribs, and at this point the wall of the thorax is thinnest, while the lung below is thickest.
Fig. 166.—The areas over which auscultation of the chest may be performed, showing their extent, position and relations to the thoracic wall. 1, Upper zone; 2, middle zone; 3, inferior zone; 4, subscapular zone.