THE HEART
The heart is placed asymmetrically, 60% or more being to the left of the midline, and extends from the second intercostal space (or following rib) to the fifth space. It thus lies mainly under cover of the limbs in an animal standing square.
The base lies in the plane of the last costochondral joint and the apex opposite the sixth cartilage, a few centimeters above the sternum; its long axis inclines somewhat caudally and to the left. Direct contact with the thoracic walls is restricted to the areas described with the lungs. The upright caudal border is related to the diaphragm and, through this, to the reticulum and liver; the sloping cranial border is related to the thymus in the young. The relations of the base include the trachea and principal bronchi, the pulmonary vessels, and lymph nodes (see Figure 27-5).The bovine heart is constructed according to the general mammalian plan and exhibits no distinctive structural features of importance. The right atrium receives a left azygous vein, by way of the coronary
Figure 27-4 Lobation and bronchial tree of the bovine lungs, schematic dorsal view. 1, Trachea; 2, tracheal bronchus; 3, right principal bronchus; 4, 4', divided right cranial lobe; 5, middle lobe; 6, right caudal lobe; 7, 7’, divided left cranial lobe; 8, left caudal lobe; 9, cranial tracheobronchial lymph node; 10, tracheobronchial lymph nodes; 11, pulmonary lymph nodes; 12, outline of accessory lobe of right lung.
sinus. It occasionally retains communication with the left atrium through an open foramen ovale; this is usually only probe patent and without significance. Two ossicles are found in the connective tissue related to the cusps of the aortic valve; they are not unique to cattle, as often supposed, but do appear to develop precociously in this species.
The left coronary artery is dominant, the right one restricted to a circumflex course. It is worth mentioning that the isthmus of the aorta (the stretch between the origin of the brachiocephalic trunk and the junction with the ductus arteriosus) is greatly constricted in the newborn calf, which is an appearance that may falsely suggest the aorta arising from the right ventricle. The usual proportions are exhibited by calves that survive birth by even a few days.The projections of the heart valves on the thoracic wall, or more accurately the puncta maxima, are obviously of much greater significance. The pulmonary and aortic valves may be regarded as being placed under the third rib and following space and the fourth rib, respectively; they are about 10 cm above the costochondral junctions, although the slope of the heart raises the aortic valve a little above and lowers the pulmonary valve a little below the suggested level. The left atrioventricular valve lies under the fourth space and fifth rib, and the right one lies under the fourth rib and space; each is at a slightly more ventral level than the associated arterial valve. It is of course only the right atrioventricular valve sound that is sought on the right side (see Figure 27-5).
Pericardiocentesis is most safety performed in the fifth intercostal space of the left side, directly dorsal to the costochondral joints.