A Laboratory Manual of the Anatomy of the Rat/The Vascular System
THE VASCULAR SYSTEM[1]
The vascular system conveys food materials and oxygen to the cells of the body, and carries away from these cells the waste products of metabolism. The food substances dissolved in the blood furnish energy for the cell's activities, and materials for replacing the protoplasm which has disintegrated during metabolism. The blood stream tends also to equalize the temperature throughout the body. Still another function is the conveyance of hormones from their sources to parts of the body whose activities the hormones accelerate or retard.
The vascular system carries two fluids—blood and lymph. The former consists of an almost colorless fluid, plasma, and of red (erythrocytes), and white (leucocytes) blood cells. When clotting occurs the plasma is resolved into the fluid serum, which is incapable of clotting under ordinary conditions, and the solid clot of fibrin. The capacity of blood to form a clot is of primary importance, since it greatly reduces the chance of death through the loss of blood following injury. The red blood cells contain hemoglobin, which has the power of combining with oxygen in the lungs, and carrying it to the tissues, where it is released. Hemoglobin gives the blood its reddish color when exposed to oxygen. There are several types of white blood cells. Some of them remove necrotic tissue or other detritus, and therefore function as scavengers. The lymph consists of plasma and white blood cells. The lymph vessels are the parts of the vascular system which transport lymph.
The heart receives blood laden with carbon dioxide (venous blood) from the systemic veins. It pumps this blood to the lungs through the pulmonary arteries. Carbon dioxide is surrendered here and oxygen enters the blood from the air chambers of the lungs. The oxygenated blood then returns to the heart through the pulmonary veins, and is propelled to the body through the systemic arteries. The branches of the arteries terminate in vessels of microscopic size, the capillaries, in which food and oxygen are surrendered to the cells of the muscles, glands, etc., and carbon dioxide enters the blood. The blood from the capillaries ultimately drains into the systemic veins and flows back to the heart. The internal structure of the heart and other details of the circulation will be discussed later.
The arteries and veins in any part of the body may often be dissected simultaneously. This is the logical procedure, since the arteries carrying blood to a part and the veins conducting blood away from it often lie close together. Considerable care must be exercised in dissecting out blood vessels, particularly veins, since they are frequently broken easily. It is better, as a rule, to locate a vessel at one place, then to follow it from that locality into the adjoining tissues just as a workman, having exposed a buried water pipe which he desires to uncover completely, follows it from the point at which it was first located. Every attempt to find a vessel anew at a place where it is concealed by other tissues unnecessarily increases the chance of breaking it accidentally.
The arteries should be injected with a colored fluid immediately after killing the rat. This fluid will force the blood out of the arteries, through the capillaries and into the veins, which will therefore be filled, after preservation, with dark coagulated blood. The fluid may be introduced through the left ventricle of the heart.
VEINS ANTERIOR TO THE HEART
Skin a rat whose blood vessels have been injected, exercising great care to remove nothing but the skin. Do not remove the integument between the hind legs on the ventral and posterior surfaces until ready to study the urinogenital system. Place the skinned animal upon its back and slit the thorax along a line parallel to the sternum but seven or eight millimeters to its left, and extending from the diaphragm to the level of the anterior end of the heart. Continue the incision dorsally along the junction of diaphragm and body wall. Expose the thoracic cavity by pressing this flap outward. The lungs and heart will be seen through this aperture. The membranous attachments of the lungs will not be injured by this dissection if the scissors are not thrust too far into the thoracic cavity. Avoid extending the incision too far forward, as the blood vessels anterior to the heart region will be cut.
Observe the membranous sac (pericardium) surrounding the heart, the large organ in the middle of the cavity. Note the mesentery-like mediastinal septum extending from the diaphragm to the anterior end of the thoracic cavity, and from the pericardium to the sternum. The small pulmonary lobe just behind the heart lies loosely in a chamber inclosed by the diaphragm behind, by lateral membranes extending from the pericardium to the diaphragm, and in front by the pericardium. The large left lobe of the lung is attached by a membrane along its dorsal border to the esophagus, or to the membrane inclosing the pulmonary lobe mentioned above. Make a second longitudinal incision in the thoracic wall seven or eight millimeters to the right of the sternum. Separate the median strip, containing the sternum, from the diaphragm and turn the strip forward. The right lung will be exposed. Observe that the dorsal edge of the extreme posterior lobe on the right side is attached by a mesentery to the esophagus, which, in turn, is similarly suspended from the dorsal wall of the thoracic cavity.
Observe, also, the thymus gland anterior to the heart. This gland has no duct. Its secretion enters the blood. It is therefore an endocrine gland, or gland of internal secretion.
The veins of mammals show considerable variation. Fluctuations from the relations described in this text should be noted. While dissecting the veins, care should be taken to avoid injuring the arteries.
Venous blood from the regions of the body anterior to the heart drains into the right and the left superior vena cava (precaval veins), which enter the right atrium of the heart. Trace one of the superior venae cavae cephalad and note that it is formed by the confluence of the subclavian and internal jugular veins. The right and left internal mammary veins pass from the ventral thoracic wall, each one parallel to and near the corresponding internal mammary artery, to the precaval vein of the same side. Both mammary arteries and veins will be seen in the median strip which has been reflected forward. The azygous vein empties into the left precava near its union with the heart. Follow the azygous dorsally at the left of the aorta, thence caudally parallel to the aorta, noting its tributaries. Is there a corresponding branch of the right precaval vein? Trace the internal jugular vein anteriorly to the thyroid gland.
The external jugular and cephalic veins unite near the middle of the clavicle to form the subclavian. The subclavian, coursing caudally, receives the blood from the anterior jugular vein, which enters the subclavian just median to the entrance of the external jugular.
Trace the external jugular vein anteriorly to its origin near the occipital region of the head. It receives blood, near its middle point, from the superficial muscles and skin of the occipital region through the posterior external jugular vein.
Cephalad to its union with the posterior external jugular, the external jugular is joined by the anterior facial vein. Trace this vein and its tributaries along the ventral and lateral regions of the head and identify them from the following description. The supraorbital vein, arising on the dorsal side of the head where it joins the superficial temporal (to be described), courses anteriorly and unites with the nasal vein at the base of the nose to form the angular vein. This runs ventrally, unites with the superior labial, and forms the anterior facial. The latter receives the inferior labial vein, continues ventrally, then caudally a short distance along the ventro-median border of the masseter muscle, and finally to its junction with the external jugular.
Carefully dissect the remaining tributaries of the external , jugular vein on the lateral side of the face, neck, and lower jaw. Identify these vessels from the following description. The superficial temporal vein, arising in the supraorbital region, passes ventrally posterior to the eye, receiving the transverse facial, which courses caudally near the zygomatic arch. Crossing the posterior root of the zygomatic arch, the superficial temporal runs posteriorly, is joined by the anterior auricular from the base of the external ear, then unites with the internal maxillary vein (described below) to form the posterior facial. The posterior facial runs through the parotid gland, and is superficial to the carotid artery. The confluence of the posterior auricular vein and the posterior facial forms the external jugular. The posterior auricular arises behind the external ear.
The internal maxillary will be found superficially on the median surface of the angle of the jaw. Here it curves laterally to join the superficial temporal, thus forming the posterior facial vein.
Exercise XIV. Make a labeled sketch of the veins anterior to the heart. This will be combined later (Exercise XXV) with a similar sketch of the veins posterior to the heart.
ARTERIES ANTERIOR TO THE HEART
The arterial blood leaves the left ventricle of the heart through the aorta. This turns dorsally and to the left, as the aortic arch, and runs caudally through the thoracic and abdominal cavities near the ventral surface of the spinal column. The thoracic and abdominal sections of the aorta are called the thoracic and abdominal aorta, respectively. The aortic arch of mammals is supposed to be homologous to the left fourth aortic arch of lower vertebrates. The branches of the aortic arch in the rat carry blood to the head, neck, fore limbs, and thoracic wall. Named in order from right to left, these branches are the innominate, left common carotid and left subclavian arteries.
The left subclavian artery runs cephalad from the aortic arch, carrying blood to the vessels described below.
Left internal mammary artery. Its origin is the left subclavian artery. It sends branches to the thymus gland, pericardium, and lung. On the inner surface of the ventral thoracic wall it turns medially to the side of the sternum and runs caudally, parallel to the internal mammary vein, giving off branches to the internal intercostal muscles. Trace it posteriorly along the dorsal surface of the rectus abdominis muscle, to which it sends branches. Several branches perforate the intercostal muscles and supply the pectoral and rectus abdominis muscles. Find the right internal mammary artery. Its branches and course are substantially the same as for the left internal mammary.
The vertebral artery upon leaving the subclavian passes cephalad about half a centimeter and enters the vertebrarterial canal at the sixth cervical vertebra. This canal consists of the transverse foramina of the cervical vertebrae. Running forward through this canal, the artery sends off branches between the vertebrae. It traverses the transverse foramen of the atlas, gives off a branch through the large oval foramen on the dorsal side of this vertebra, passes through the atlantal foramen, and enters the cranial cavity by way of the foramen magnum.
Thyro-cervical trunk. It leaves the subclavian artery medial to the first rib and passes anterolaterally to a position dorsal to the proximal end of the clavicle, where it divides into several branches. These branches supply glands on the surface of the neck, and muscles of the neck region.
Costo-cervical trunk. This vessel leaves the subclavian artery at the same level as the internal mammary, courses laterally anterior to the first rib, then sends branches to the lateral muscles of the thorax and deeper muscles of the neck.
The axillary artery is the continuation of the subclavian lateral to the first rib. Its branches are as follows.
(1) Lateral thoracic artery. It proceeds caudally from the axillary artery along the medial surface of the cutaneous maximus muscle.
(2) Subscapular artery. It arises from the axillary artery and divides into branches which supply muscles in the upper arm and thorax. (3) Deep artery of the arm. Leaving the distal end of the axillary artery, it sends branches to muscles of the upper arm (triceps), then proceeds distally along a spiral course, and finally sends branches down the anterior and lateral surfaces of the fore arm.
(4) The brachial artery, the continuation of the axillary, passes through the arm pit region, thence down the medial surface of the upper arm in company with the large nerves in this part of the arm. It lies throughout its length close to the posterior edge of the biceps brachii muscle. The artery provides branches for the muscles of the elbow region, but the main trunk passes down the forearm.
The second large arterial trunk leaving the aortic arch is the left common carotid artery. It originates from the aortic arch between the innominate artery and the origin of the left subclavian, and passes forward lateral to the trachea as far as the anterior end of the neck, where it splits into the external and internal carotid arteries.
The internal carotid artery proceeds anterodorsally from its origin and divides into two branches immediately posterior to the tympanic bulla. The dorsal branch of the two enters the tympanic bulla through the posterior lacerated foramen, which is situated on the median surface of the bulla immediately in front of the jugular process. The ventral branch enters the cranium by way of the carotid foramen.
The external carotid artery gives off the following branches.
(1) The occipital artery courses anterodorsally, from its origin immediately anterior to the internal carotid artery, passes around the median surface of the jugular process, thence dorsally to the dorsal musculature of the neck.
(2) The superior thyroid artery runs ventrally from its origin cephalad to the occipital artery, then posteriorly, sending branches to the thyroid gland, and one small branch to the larynx.
(3) The lingual artery leaves the external carotid anterior to the superior thyroid artery, enters the posterior region of the tongue, and follows a sinuous course nearly to the tip in the ventral part of that organ.
(4) The external maxillary or facial artery leaves the external carotid a short distance anterior to the lingual artery, and passes anteroventrally along the medial surface of the masseter muscle. Anteriorly it occupies the groove between the masseter and digastric muscles. It bifurcates about half a centimeter back of the angle between the upper and lower lips. The dorsal ramus supplies the snout and lateral wall of the mouth. The ventral ramus supplies the lower lip. This artery sends vessels to the lymph and salivary glands of the neck region, to the masseter muscle, the ventral musculature of the lower jaw, and the floor of the posterior region of the buccal cavity.
(5) Posterior auricular artery. The external carotid artery turns abruptly dorsally immediately after giving off the external maxillary. It turns anteriorly again ventrolateral to the tympanic bulla. The posterior auricular artery arises at the latter turn. It courses dorsally along the posterior surface of the external auditory meatus to the pinna of the ear, to which it sends branches. It also supplies the muscles adjoining the pinna. Trace the terminal branches of the external carotid.
The innominate artery is the first arterial trunk leaving the aortic arch. A short distance from its origin the innominate divides into the right common carotid and the right subclavian arteries. The subclavian passes anterolaterally to the anterior surface of the first rib, where, in company with the nerves of the brachial plexus, it leaves the thoracic cavity and becomes the right axillary artery.
The right internal mammary artery comes off from the posterior surface of the subclavian just median to the first rib. Its course is described in connection with the branches of the left subclavian.
The branches of the right axillary artery closely resemble those of the left. Verify this by a dissection of the vessels of the right arm.
Exercise XV. Sketch and label the arteries anterior to the heart. This drawing will be combined later (Exercise XXVI) with a similar sketch of the arteries posterior to the heart.
THE HEART
Study the attachments of the pericardium, the transparent membrane which surrounds the heart. If the thorax has been opened with sufficient care the pericardium will be unruptured. Insert a blowpipe through a small incision in the pericardium and inflate it with air. The pericardial cavity the space between the pericardium and heart, is a part of the body cavity or coelom. The two other components of the coelom are the thoracic and abdominal cavities.
Remove the pericardium and study the heart in position. Note the attachments of the aortic arch, pulmonary arteries and veins, the superior venae cavae, and the inferior vena cava. The heart contains four chambers — two anterior 'atria and two posterior ventricles. The atrial and ventricular portions of the organ may be identified easily in a preserved animal. The atria are very dark in color because they contain masses of clotted blood, while the ventricles are decidedly lighter. The coronary sulcus is the deep furrow on the heart's surface which separates the atria from the ventricles. The right and left auricles (au- ricular appendices) are the ventral extensions of the right and left atria, respectively, which extend ventrally on each side of the pulmonary artery. Notice on the surface of the heart the injected coronary artery, which carries blood to the tissues of the heart, and the coronary veins, which drain the venous blood back into the general circulation.
The heart is a muscular pump which propels the blood throughout the body. Its complete failure to function means almost instant death. Venous blood returning from the capillaries of the muscles, brain, digestive organs, kidneys, etc. of the body enters the right atrium through the superior and inferior venae cavae. Contraction of the right atrium forces the blood into the right ventricle. The two atria contract simultaneously, but their contraction alternates with the simultaneous contraction (systole) of the ventricles. When the right ventricle is full it drives the blood through the pulmonary artery to the lungs, then relaxes (diastole) to receive more blood from the atrium. The excess of carbon dioxide carried by the blood is eliminated in the lung capillaries into the air contained in the minute air sacs (alveoli) of the lung. At the same time oxygen from the alveolar air penetrates the walls of the air sacs and capillaries and unites chemically with the hemoglobin, or red coloring matter of the blood. The blood then traverses the pulmonary veins to the left atrium of the heart, thence to the left ventricle, and finally out through the aorta to the tissues of the body. The internal mechanism of the heart may now be elucidated by an examination of the dissected organ.
Insert one point of the scissors into the severed end of the pulmonary artery, carefully slit it lengthwise, continuing the cut along the lateral wall of the right ventricle. Bend back the cut walls and carefully wash out the coagulated blood. Observe the shape of the right ventricular cavity. It communicates with the pulmonary artery through the pulmonary orifice, which is surrounded by the three pocket-like segments of the pulmonary valve. Locate these valves and determine how they function. The right atrioventricular, or tricuspid, orifice is the aperture through which blood enters the right ventricle from the right atrium. It is guarded by the tricuspid valve. The inner surface of the ventricle bears muscular protuberances (trahecidae carneae), some of which (the papillary muscles) are attached to the tricuspid valves by slender strands, the chordae tendineae. When the ventricle contracts the blood starts to rush back into the atrium, but in so doing the tricuspid valve is thrust across the aperture by the blood stream. The edges of the valve would be forced up into the atrium were it not for the chordae tendineae and the papillary muscles, which hold the flaps athwart the opening. Thus blood having once entered the right ventricle is unable to retreat into the atrium when the ventricle contracts, but must find its exit through the pulmonary artery. Insert the scissors' point into the severed inferior vena cava and open the right atrium by a cut extending forward to the right superior vena cava along the anterior surface of the atrium. Locate the openings of the superior and inferior venae cavae, and the atrioventricular aperture. Find the membranous valves near the entrance of the superior vena cava.
Slit the ventral wall of the left ventricle lengthwise by inserting one point of the scissors into the cut end of the aorta. Extend the incision to the .apex of the heart. Try not to injure the aortic valve whose semilunar segments surround the opening of the ventricle into the aorta. The muscular wall of the left ventricle is considerably thicker than the wall of the right, for more pressure is required to send blood throughout the body than through the pulmonary circulation. Observe the trabeculae carneae on the inner surface of the left ventricle's wall. The aortic orifice, surrounded by the aortic valve, is the opening into the aorta. Part of the valve may have been torn in opening the ventricle. The mitral, or bicuspid valve surrounds the mitral orifice, through which blood passes from the left atrium to the left ventricle. Note the chordae tendineae extending from this valve also to the ventricular wall. The ventricular septum is the muscular partition between the cavities of the right and left ventricles. Open the left atrium, wash out the coagulated blood, and find the orifices of the pulmonary veins. How many openings are there? View the partition between the atria (the atrial septum) by strong transmitted light. The fossa ovalis is the translucent area which will be seen in this septum. Ask the instructor to explain its significance.
Exercise XVI. Make a diagrammatic sketch of the internal anatomy of the heart.
The arteries and veins posterior to the heart will be studied after the examination of the respiratory, digestive, and urinogenital systems.
- ↑ The discussion of the vascular system is divided into three parts, separated from one another by descriptions of other systems. Experience has shown the convenience of studying the heart and blood vessels anterior to it early in the dissection, of examining the hepatic portal system immediately before the abdominal organs are observed, and of postponing the analysis of the vessels posterior to the heart until after the study of the abdominal contents.