Tree shrew heart and thorax
Photo and Text: Terence Mitchell
Reflect muscles - Use a probe to define the borders of all muscles covering the rib cage. Cut any medial attachments and reflect these muscles laterally. You should now see only ribs and intercostal muscles covering the thoracic cavity.
Reflect the rib cage - Insert the tip of a pair of scissors between the first rib and clavicle and cut the sternum. Cut laterally as far as you can without damaging any external structures. Be sure to keep the lower blade of the scissors pressed against the rib cage to keep from disturbing the viscera. Now make an anteroposterior cut down the side of the animal keeping the incision as dorsal as possible. Stop when you hit the last rib. Repeat for the other side. Reflect the rib cage inferiorly.
Lungs - After removing the rib cage you will see a heart in the midline of the body with a large thymus gland situated on its anterior surface. A thick layer of tissue known as the pericardium surrounds the heart. A right and left lung are located on either side of the heart. The lungs are covered in a layer of visceral pleura and surrounded by another layer attached to the thoracic cavity. The potential space between these two layers is known as the pleural cavity.
Remove the parietal pleura to get a better view of the lungs. Mobilize the lungs so that they are only attached via the bundle of arteries, veins, and bronchus connecting them to the heart and trachea. Grasp this bundle as close to the root of the lung as possible and cut with scissors.
Note that the larger right lung has four lobes - inferior, superior, middle, and azygous. The azygous lobe wrapped and abutted the dorsal side of the heart. The smaller left lung has just three lobes - inferior, superior, and middle. Each lobe is supplied by its own secondary bronchi arising from one of the two primary bronchi of the trachea.
Heart - The heart is surrounded by a thin layer fascial layer known as the pericardium. Remove this layer to better expose the heart. The heart lies almost vertically in the midline of the body. The atria are located on the dorsal side with the auricles showing ventrally. The apex of the heart is round, blunt, and composed about equally of the left and right ventricles. The ventral view of the heart is taken up mostly by the right ventricle. There are several large vessels entering the heart. The aorta runs cranially for a few millimeters and then arches to the left. The right and left superior vena cavae run cranially on either side of the heart. Note the large azygous vein merging with the left superior vena cava. The inferior vena cava can be seen extending caudally from the inferior aspect of the heart. These vessels are discussed in more detail below.
Cut the right and left superior vena cava, azygous vein, inferior vena cavae,
and aorta. Cut each vessel close to the heart - leaving enough of a trunk that
you can still identify each structure. Gently remove the heart with a pair of
forceps. Cut any remaining fascial layers. Use the sharp forceps and probe to
clean out the fat and muscle obscuring your view of the major vessels. Take
care not to damage the vessels themselves.
Thoracic cavity - After removing the heart you should be able to see several structures of the thoracic cavity. First examine the ringed trachea descending from the pharynx, and giving rise to the right and left primary bronchus. Next, note the muscular diaphragm defining the inferior border of the thoracic cavity (See Figure 1). The diaphragm is pierced by the inferior vena cava to the right of the midline and the esophagus to the left. The phrenic nerves innervate the diaphragm on either side of the inferior vena cava. Several of the major arteries and veins will also be visible at this time. The vagus is a fine cord running along the esophagus. Finally, the sympathetic trunk runs along the posterior of the thoracic wall just dorsal to the esophagus.
The aorta and its branches - The aorta arises from the left ventricle of the heart and runs anteriorly for a few millimeters before arching posteriorly to the left. In Figure 1 you can see the brachiocephalic trunk splits off from the arch and then immediately bifurcates into a right common carotid artery and a right subclavian artery. Next the right common carotid artery and right subclavian artery originate from the arch. The descending aorta then gives off several intercostal branches as it runs down the length of the thoracic cavity to the left of esophagus. It finally leaves the diaphragm by exiting through the aortic hiatus well to the left of the esophagus. The common carotid artery splits into the internal and external carotid near the base of the skull. Both subclavian arteries run anterolaterally towards the upper limbs, giving off a vertebral artery near the neck. After passing the first rib they continue on as the axillary artery and pass into and supply the upper limb as the brachial artery.
The superior vena cava and its tributaries - The tree shrew has right and left superior vena cavae, both of which enter into the right atrium. The course and branches of these veins are symmetrical except that the left receives the azygous vein. The left superior vena cava runs laterally over the dorsum of the heart before terminating in the right atrium. The azygous vein arches superiorly over the root of the lung and then follows a course deep to the aorta, and exits the thoracic cavity aorta through the aortic hiatus - see figure.
Pulmonary structures - It was difficult to locate the origin of the pulmonary artery and vein from the right ventricle and left atrium. However, it was possible to find these structures in the cut end which had been attached to the lungs. On each side I traced back the pulmonary veins to the left atrium and the pulmonary arteries to the right ventricle. Both the right and left pulmonary arteries join to form a common pulmonary trunk. There appeared to be several pulmonary veins on each side. I could not count the exact number as these structures were damaged in the removal of the lungs. However, all vessels on both sides merge near the left atrium to form a single common trunk. I could also trace the primary bronchus back to the trachea from the root of the lung.
The lungs of some mammals can function for more than just respiration. Some cursorial mammals use their lungs for mechanical support during locomotion. Since most cursorial animals undergo large decelerations during each stride the heart tends to be compressed anteriorly [1]. In many such mammals, there is a cranial extension of the right lobe between the heart and the sternum, which serves to cushion the viscera from excessive forces. The pattern of branching inside the lungs of cursors and non-cursors is the same, and an equivalent volume of lung tissue could be created without this odd extension if it were for solely respiratory purposes. This pattern is similar to the lung morphology in tree shrews. In tree shrews however there is a fourth lobe, azygous, of the right lung that wraps around the posterior and dorsal aspects of the heart [2]. The intimate contact of this lobe with the heart, and its odd asymmetry suggest that it too may be functioning as a mechanical cushion. It could perhaps stabilize the heart during vertical climbing. It would certainly be interesting to examine the distribution of this trait among ecologically similar taxa.
1 Simons, R.S. 1996. Lung Morphology of Cursorial and Non-Cursorial Mammals:
Lagomorphs As a Case Study for a Pneumatic Stabilization Hypothesis. J. Morphol.
230:229-316.
2 Le Gros Clark, W.E. 1926. On the anatomy of the pen-tailed tree shrew (Ptilocercus lowii). Proc. Zool. Soc. Lond. 1926:1179-1309.
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