The Thorax
2.13 Heart (Cor)
Goal: Know the atrial and auricular sur- faces of the heart. Use the cow skeleton to identify the rib boundaries for listening to the heart sounds. Understand that the heart in the live animal lies between the plane of the 3rd and 5th intercostal space, mostly to the left side of the thorax.
The area for heart sounds is therefore mostly covered by the forelimb. Your instructor, depending on your knowledge of the subject, will determine the level of details required.
Build a mental picture of the location of the heart in situ. On an isolated heart, examine the exterior and interior structures of the heart including the fibrous sac covering
the heart (pericardium), heart chambers, and heart valves.
The heart lies in the middle mediastinum mostly to the left side of the median plane.
The base of the heart faces dorsally and the apex lies ventrally close to the sternum.
Within the mediastinum, the heart is sur- rounded by the pericardium.
2.13.1 Pericardium
The pericardium is the sac that surrounds the heart. It has both fibrous and serous layers, and a paper‐thin pericardial cavity that contains a small amount of serous fluid.
Superficially, the pericardial sac is intimately surrounded by a serous membrane, the pari- etal mediastinal pleura (Figure 2.14).
The serous pericardium is sometimes divided into parietal layer of the serous pericardium and visceral layer of the serous pericardium (Figure 2.14b). The parietal layer of the serous pericardium is adhered to the inner layer of the fibrous peri- cardium. The visceral layer of serous pericar- dium is the epicardium or visceral layer of the serous pericardium.
With the help of Figure 2.14, study the layers that make up the pericardium. As you can see, the fibrous pericardium is interposed between the mediastinal pleura and the parietal layer of the serous pericardium.
From superficial to deep, the pericardium includes the fibrous pericardium (the strongest layer of the pericardium), the pari- etal layer of the serous pericardium, and finally the visceral layer of the serous peri- cardium closely adhered to the heart wall.
The visceral layer of the serous pericardium is also called the epicardium. A space (peri- cardial cavity) lies between the parietal and visceral serous pericardium. It has a small amount of fluid that helps in friction‐free contraction of the heart.
Understand that the mediastinal pleura, the fibrous pericardium, and the parietal layer of the serous pericardium are closely fused to each other and are inseparable.
Clinicians often refer to these fused layers as the pericardium.
The extension of the fibrous pericardium, sternopericardial ligament, anchors the heart to the sternum. This ligament is called phrenicopericardial ligament in dogs because it attaches the more obliquely ori- ented dog heart to the diaphragm instead of the sternum.
2.13.2 External Features of the Heart
Study features of the heart on an isolated fresh or preserved (formaldehyde fixed or plastinated) specimen.
The heart has four chambers (two atria and two ventricles) separated by interatrial and interventricular septa.
Topographically, the left surface of the heart is called the auricular surface (the view with two auricles seen hugging the pul- monary trunk). The heart is rotated about its vertical axis to the left side with the right auricle seen on the left side. The right surface is the atrial surface.
The heart has a wide dorsal base and a ventral tapering end apex. The apex is formed only by the left ventricle.
With help of Figure 2.15, study the follow- ing superficial features of the heart:
1) Atria (left and right). Use your probe to identify the cranial and caudal vena cavae entering the right atrium. The left atrium receives several pulmonary veins. They bring relatively oxygenated blood from the lungs to the heart.
Box 2.8
The fibrous pericardium is the strongest layer of the pericardium. Although mild dis- tortion in the pericardium occurs during heart cycle, the fibrous pericardium pre- vents significant changes or distortion in heart shape that can occur in heart contrac- tion or in heart diseases where excessive fluid could accumulate in the pericardial cavity.
2) Auricles (left and right). The auricles are blind extensions of the atria that provide additional space for blood and have an endocrine function.
3) Ventricles (left and right), the left ventricle is thicker and forms the apex of the heart.
4) Grooves:
● Paraconal interventricular groove:
lies caudal and parallels to the conus arteriosus on the auricular surface of the heart, hence the name. The paraconal ventricular branches of the
Brachiocephalic trunk
Pulmonary
trunk Aorta
Fibrous pericardium
Remnant of sternopericardial ligament
Ox
(a)
(b)
Heart (cor)
Pericardial cavity
Mediastinal pleura
Visceral layer of serous pericardium (epicardium)
Parietal layer of serous pericardium
Fibrous pericardium
Fused three layers
Figure 2.14 (a) Isolated bovine heart (auricular surface). Figure shows fibrous pericardium covering the auricular (left) surface of the heart. (b) Schematic illustration of the layers of the pericardium. Understand that the layers of the pericardium form a sac around the heart. The fibrous pericardium extends ventrally from the apex of the heart to form the sternopericardial ligament that anchors the heart to the sternum (not illustrated). The outer three layers (with the fibrous pericardium being the thickest) are fused and are inseparable. The pericardial cavity is a very small capillary space filled with small amount of serous fluid. The visceral layer of the serous pericardium (or the epicardium) adheres closely to the wall of the heart.
left coronary artery and the great car- diac vein course within this groove.
● (Subsinuosal) interventricular and intermediate grooves: lies on the cau- dal aspect of the heart below the coro- nary sinus, hence the name. The subsinuosal interventricular branch of the left coronary artery and middle car- diac vein course within the subsinuosal groove. An intermediate groove is found in close proximity to the subsinuosal
groove in the caudal aspect of the heart in ruminants. The caudal cardiac vein and an intermediate branch of the left coronary artery courses into it.
● Coronary groove: separates the atria from the ventricles. The left and right coronary arteries course within the cor- onary groove.
5) Conus arteriosus: The portion of the right ventricle as it approaches the pul- monary trunk is shaped like a cone, hence
Pulmonary trunk Right auricle
Left auricle Aorta
Brachiocephalic trunk
Right ventricle Paraconal
interventricular groove with liked named a. and great cardiac v.
Left ventricle Ligamentum arteriosum
Apex (a)
(b)
Left azygos v.
Cranial vena cava
Ox
Cranial vena cava Brachiocephalic trunk Right atrium
Pulmonary vv.
Coronary groove and right coronary a.
Right ventricle Aorta
Pulmonary a.
Caudal vena cava Left ventricle Subsinuosal interventricular groove with like- named artery and middle cardiac v.
Ox
Figure 2.15 Bovine heart. External features of the heart on (a) auricular (left) surface; (b) atrial (right) surface.
the name conus arteriosus of this region of the heart. Briefly, the conus arteriosus is defined as the outflow tract of the right ventricle.
6) Pulmonary trunk and pulmonary arter- ies: The pulmonary trunk carries relatively deoxygenated blood from the right ventri- cle toward the lungs. It branches into left and right pulmonary arteries. Identify the ligamentum arteriosum between the pul- monary trunk and the aortic arch.
7) Aortic arch: The aorta in the thorax is divided into ascending aorta, aortic arch, and descending aorta (Figure 2.11). The left and right coronary arteries that sup- ply the heart wall originate from the ascending aorta. No need to identify the ascending aorta. The aortic arch gives one large vessel, the brachiocephalic trunk.
8) Brachiocephalic trunk: The brachioce- phalic trunk is the only vessel originating from the aortic arch in ruminants and horses. It gives four vessels (discussed earlier).
9) Ligamentum arteriosum, a fibrous structure connecting the pulmonary trunk with the aortic arch. It is the rem- nant of a fetal shunt that connects these two vessels.
2.13.3 Interior of the Heart
Goal: With the help of Figures 2.16 and 2.17, study the interior of the heart on transverse and vertical sections of the right atrium and right ventricle, respec- tively. Study the heart valves on a trans- verse section cut at the level of the coronary groove.
With the help of Figure 2.16, open the right atrium by making a transverse incision from the cranial vena cava across the lateral wall of the atrium to the caudal vena cava. Expand the ventral incision to the level of the right atrioventricular valve and to the right auricle.
With the help of Figure 2.16 and the fol- lowing discussion, identify the intervenous tubercle, fossa ovalis, coronary sinus, foramina for the cranial and caudal vena cava, and pectinate muscles on the right atrium.
2.13.3.1 Structures of the Right Atrium
● Intervenous tubercle: a vertical ridge that courses midway between the open- ings of the cranial and caudal vena cavae
Intervenous tubercle
Fossa ovalis
Coronary sinus
Right atrium
Ox
Figure 2.16 Bovine heart. Right atrium opened to show interior structures.
(Figure 2.16). The intervenous tubercle is thought to direct the blood from the caval openings ventrally toward the right atrio- ventricular valves, thus decreasing blood turbulence that might have resulted from the collision of the two blood streams.
● Fossa ovalis: a depression behind the inter- venous tubercle. It faces the opening of the caudal vena cava. The fossa is a remnant of the foramen ovalis in the fetus that connects the right and left atrium (Figure 2.16).
● Coronary sinus: look for the coronary sinus opening below the opening of the cau- dal vena cava (Figure 2.16). The coronary sinus is where the left azygos, middle, cau- dal cardiac, and great cardiac veins empty.
● The great cardiac vein: one of the veins that drain the heart muscles. It opens into the coronary sinus. Look for the great car- diac vein coursing in the paraconal inter- ventricular groove and coronary groove on the auricular surface (Figure 2.15a).
● Pectinate muscles: long round muscular bands occupying the interior wall of the auricles (Figure 2.17). Some are present in the atrial chamber. They reinforce the wall of the auricle.
2.13.3.2 Structures of the Right Ventricle With the help of Figure 2.17 and the follow- ing discussion, identify the chordae tendineae, papillary muscles, cusps for atrioventricular valves, right atrioventric- ular valve, trabecula septomarginalis, interventricular septum, and trabeculae carneae.
Understand that the internal structures in the right ventricle are also present in the left ventricle. We will identify ventricular internal structures on the right ventricle because it has a relatively thin wall and larger cavity com- pared with the left ventricle. Identify the fol- lowing structures within the right ventricle:
2.13.3.2.1 Right Atrioventricular Valve
The right atrioventricular valve (R‐AV) regu- lates venous blood flow from the right atrium into the right ventricle where backflow is prevented during systole and permitted dur- ing diastole. This valve is also called tricus- pid valve. Likewise, a left atrioventricular ventricular (mitral) valve permits the flow of arterial blood from the left atrium to the left ventricle during diastole and prevents backflow during systole.
Aorta Pulmonary trunk
Left AV (Parietal cusp) Chordae tendineae Papillary m.
Trabeculae septomarginale
Left ventricle Right ventricle
Right auricle Origin of left coronary a.
Trabeculae carneae Brachiocephalic trunk
Apex Pectinate mm.
Left auricle
Ox
Figure 2.17 Bovine heart: sagittal section. Note the difference in wall thickness of the right (thin) and left (thick) ventricles. The term trabeculae septomarginale is plural for trabecula septomarginalis.
2.13.3.2.2 Chordae Tendineae, Papillary Muscles, and Cusps of the Right Atrioventricular Valve The chordae tendineae are thin fibrous cords that anchor the cusps of the atrioven- tricular valves to the tips of papillary mus- cles on the ventricular wall. This arrangement is likened to a parachute (Figure 2.17). The atrioventricular valve has parietal and septal cusps (flat connective tissue sheets) that close the atrioventricular opening.
The chordae tendineae help prevent ever- sion of the cusps dorsally into the atria dur- ing heart contraction (systole).
2.13.3.2.3 Trabecula Septomarginalis
The trabecula septomarginalis is a long cir- cular muscular cord that transversely crosses the lumen of the ventricle. It extends from the interventricular septum to the lateral (parietal) wall of the ventricle. The origin on the septal wall is usually from a papillary muscle.
The trabecula septomarginalis contains of Purkinje fibers that aid with fast signal conduction (quick detour) across the heart wall.
2.13.3.2.4 Ossa Cordis
The heart skeleton of adult ruminants (cat- tle, goats, and sheep) has two areas of
ossification around the right and left semilu- nar cusps of the aortic valve known as ossa cordis (Figure 2.18). Sheep only have a sin- gle os cordis. These small bones support the aortic valve.
2.13.3.2.5 Coronary Arteries
The heart wall receives blood from the left and right coronary arteries. They are located within the left and right coronary grooves, respectively. The branching of the left and right coronary arteries in ruminants is similar to that of the dog but slightly differ- ent from the horse.
The substantial left coronary artery origi- nates from the left aortic sinus just dorsal to the aortic valve. The left coronary artery gives rise to septal, paraconal interven- tricular, circumflex, intermediate, and subsinuosal interventricular branches. The septal artery is deeply located at the origin of the left coronary artery from the aorta. The septal artery arborizes deeply into the inter- ventricular septum. It will not be identified.
The right coronary artery originates from the right aortic sinus. The right coronary artery is distributed mainly to the wall of the right ventricle with only a circumflex branch coursing within the right coronary groove.
Left atrioventricular valve
Pulmonary valve Aortic valve
Right atrioventricular valve
Ossa cordis
Ox
Figure 2.18 Bovine heart valves. Transverse section of the bovine heart at the level of the coronary groove:
dorsal view. Singular for the term ossa cordis is os cordis.
Clean the fat from the coronary groove (Figure 2.15a) to identify the left and right coronary arteries and their branches.
On a transverse section of the heart, iden- tify the aortic and pulmonary semilunar valves, and right (tricuspid) and left (mitral) atrioventricular valves (Figure 2.18).