In the large animal, the thoracic cavity is much smaller than the thoracic region and many factors contribute to this. On the skeleton observe the long spinous process on the cranial thoracic vertebrae. The thoracic limbs of the animal are responsible for carrying approximately 60% of the body weight and the thoracic limb has no direct boney connection to the trunk. Consequently, the muscles of the synsarcosis need additional surface area for attachment.
In all domestic animals the diaphragm projects cranially to the 6th rib or 6th intercostal space. This results in an intrathoracic region of the abdominal cavity. This is most exaggerated in the horse that has 18 pairs of ribs, because the cranial projection of diaphragm still makes the 6th rib or 6th intercostal space. All of this leads to a relatively smaller thoracic cavity than the region would suggest.
The area of pulmonary auscultation is much smaller than the space of the thoracic cavity would indicate. First, the lungs do not completely fill the pleural cavity. Second, the cranial portion of the thoracic cavity is mostly excluded from auscultation because of the thoracic limb. Another factor that reduces the area of pulmonary auscultation is the relatively thin margins of the lung extending around the diaphragm coupled with the thickness of the body wall and hair coat. This results in a thin basilar margin of lung providing little value for auscultation.