Dissection of the Circulatory System 117
I. The Heart (P. 274, and Figs. 115-)
Use the heart from the specimen dissected for the muscles, or from the specimen on which the viscera were studied.
1. Study the outside (p. 275, and Figs. 115 and 116), and learn to recognize all parts. Find the pulmonary veins (p. 275, and Fig. 116, g, h, i) and cut them, thus separating the heart from the lungs.
2. The Pericardium (p. 279). In a specimen the thoracic contents of which have not been injured, dissect the mediastinal septum from the pericardial sac and remove fat about the great blood-vessels so as to expose them all fully. Study their relations. Prick and inflate the pericardium. Slit it lengthwise over the ventricles and reflect it so as to expose its contents. Study the attachment of serous and fibrous layers to the heart and their relation to one another.
3. In dissecting the heart follow the course of the blood, studying each cavity with the aid of the descriptions (p p. 275-279) as you proceed. Dissect as follows:
a. Remove the dorsal wall of the right auricle (p. 275) and of its appendage except that part of it to which the venæ cavæ are attached.
b. Introduce the probe from the right auricle into the right ventricle, and feel with the probe the line along which the ventricular wall joins the septum. Cut along this line so as to turn back the ventricular wall as a flap, which remains attached at the base of the ventricle.
c. Introduce a probe through the conus arteriosus into the pulmonary artery and cut along the probe.
d. Remove the dorsal wall of the left auricle and its appendage, but do not remove that part to which the pulmonary veins are attached.
e. Make a longitudinal incision beginning at the apex and divaricate the lips of the cut as you pass toward the base, thus avoiding injury to the lateral flap of the bicuspid valve. Without injury to the flap or the columnæ carneæ, remove the heart-wall at the sides of this incision near the base of the ventricle, as much as necessary to expose the cavity.
f. Pass a probe into the aorta. Introduce scissors behind the septal flap of the bicuspid valve and slit the aorta without injury to the bicuspid valve.
II. Dissection of the Blood-vessels
Preparation and Injection.—A new specimen must be prepared for the dissection of the blood-vessels. The same specimen may be used, if necessary, for the dissection of the peripheral nervous system,—one side being used for the blood-vessels, the other for the nerves,—but it is much better to use separate specimens for the two systems.
Inject the femoral artery of the specimen with either five per cent. formalin, or with the mixture of five per cent. formalin and glycerine, exactly as for the muscles. Then, without removing the canula or waiting, inject the artery with red starch. This is prepared as follows:
Mix together equal volumes of vermilion (or red lead), glycerine, and five per cent. formalin. Grind these together in a mortar, so as to destroy the lumps; strain the resulting mixture through fine muslin. This color mixture can be preserved in a closed bottle till it is to be used. Then mix together one volume of dry starch, one and one-fourth volumes of five per cent. formalin, and one-fourth volume of the color mixture. See that no lumps are present.
Inject the above red starch mass into the femoral artery. It will drive into the veins the formalin already injected, and the arteries will be filled with the red starch.
The veins need not be injected, as they will be found to be filled with the formalin mixture, colored by the blood, so that they can be traced without great difficulty.
Dissection of Blood-vessels.—In general the arteries and veins will be traced together. It is an excellent plan to make a sketch of each vessel as it is dissected, showing its origin, branches, and name. This will be a great aid to the memory and will be of much assistance in reviewing the vessels. The sketches can later be combined into a well-ordered drawing. The description of each vessel must of course be studied as the vessel is dissected.
In tracing blood-vessels do not grasp them between the forceps-blades, but handle them by taking hold with the forceps of the connective-tissue coats of the vessels. Do not use the scalpel, but pull away the connective tissue and fat with fine forceps, using two pairs of forceps or one forceps and the tracer.
Variations are especially common in the blood-vessels, and the student must not be surprised to find considerable deviations from the conditions described and figured in the text. These variations usually present nothing new in principle and are easily understood by comparing them with the structures described.
Directions for Dissecting the Blood-vessels.—Make a median longitudinal incision through the skin from the symphysis of the jaw to the caudal end of the xiphoid process. About three or four centimeters caudad of the cranial end of the sternum make an incision at nearly right angles to this, passing from the first incision on the ventral side of the left arm about to the elbow. Reflect the flaps of skin, so as to uncover the left side of the thorax and the under surface of the arm, exposing the pectoral muscles. Isolate and transect the pectoral muscles one at a time, cutting each near its thoracic attachment. (The muscles (p. 145) should be reviewed at the same time.) In this way the nerves and blood-vessels of the axilla are exposed (Fig. 122, p. 295).
Find the axillary artery and vein (Fig. 122, f and g) emerging from the thorax just craniad of the first rib, along with the nerves of the brachial plexus. Remove connective tissue, etc., so that the vessels and nerves are well isolated as they pass out of the thorax. Take great pains not to puncture the vessels, particularly the veins.
Then remove the left side of the thorax by cutting through the first rib near its sternal end and then near its dorsal end, without injury to the vessels and nerves, cut the other ribs in the same way, and take out the thoracic wall.
Now find with tracer and forceps the great blood-vessels leaving the cranial end of the heart (see Fig. 118). Take the greatest pains not to injure them. Find the aorta and aortic arch (p. 281); the left subclavian artery (p. 283) (continuous with the axillary); the innominate artery (p. 282), and the beginnings of its three branches (see Fig. 115). Find also the superior vena cava, the innominate veins, and the subclavian vein, continuous with the axillary vein.
I. Study the smaller branches of the thoracic aorta (p. 283, and Fig. 118)—the intercostals, the bronchial and œsophageal arteries, and the first pair of lumbar arteries. (The coronary arteries will be examined later.)
II. Dissect the subclavian and its branches (p. 290) as follows:
1. The internal mammary (p. 292). Follow it onto the ventral wall of the abdomen. Follow the vein at the same time (p. 318).
2. The vertebral artery (p. 291). Find its beginning, but do not trace it at present.
3. The costocervical axis (p. 292). Find its beginning, and trace the superior intercostal branch some distance The other branches are not to be followed at present.
4. The thyrocervical axis (p. 293). Find its beginning, but do not trace it at present.
5. The axillary artery (continuation of the subclavian) (p. 294). Follow its branches, tracing at the same time the axillary vein (p. 318). (Consult Fig. 122.) In tracing the blood-vessels, separate the muscles, but do not cut them except where absolutely necessary. (The muscles should be reviewed as the vessels are traced.)
The following notes may be of assistance in following the different branches:
(a) The anterior thoracic was probably cut in dissecting the pectoral muscles; it may be found, but its distal end is probably cut off.
(b) The long thoracic is easily followed.
(c) The subscapular. Follow the main artery before dissecting its branches. Where the subscapular disappears between the long head of the triceps, the latissimus dorsi, and the scapula, it may be traced and found again as follows: Remove the skin from the outer side of the shoulder,—taking great pains to remove only the skin and not to injure the vena cephalica (p. 319), a large vein that lies just beneath the skin on the lateral surface of the shoulder, coming from the elbow. The branches of the subscapular will be found appearing on the lateral surface of the arm in the angle between the spinotrapezius, the long head of the triceps, and the infraspinatus. The distal branches may then be followed.
(d) The posterior circumflex (p. 296) may be traced distad in a similar manner, by seeking it beneath the caudal border of the spinotrapezius. (Do not injure the vena cephalica.)
(e) The other branches of the brachial artery and vein present no difficulty till we come to the collateralis radialis superior (Fig. 122, x). This must be traced with great care, along with the vena mediana cubiti (p. 319, and Fig. 122, y). Remove the skin from the extensor side of the forearm, taking great pains not to remove anything more than the skin. The artery and vein lie beneath the skin and should be traced to the hand (see Fig. 130).
Along with the collateral radial artery trace the vena cephalica (p. 319, and Fig. 130, a and c). Follow it across the shoulder, noting the branch to the posterior circumflex vein.
The remainder of the blood-vessels of the arm present no special difficulty (see Figs. 123 and 124).
Make a diagram of the subclavian artery, as far as dissected.
III. Remove the skin from the sides of the neck, exposing the sternomastoid muscles and the external jugular veins crossing them (see Fig. 131). Clean the surface of the sternomastoid muscles, without injuring the vein; separate the two muscles caudad, and cut each close to the attachment to the sternum. Find the sternohyoid and sternothyroid muscles, and cut them close to their attachment to the first rib. Uncover the right side of the thorax in the same way as the left, cut the ribs without injury to the nerves and vessels of the right axilla, cut the internal mammary artery and vein (after tying the latter), and thus remove the sternum with nearly the entire thoracic wall.
The blood-vessels of the thorax may now be more completely exposed. (If the nerves are to be dissected on the same specimen, find the phrenic, vagus, and sympathetic nerves (Fig. 157), and take the greatest pains not to injure them.)
1. The coronary arteries and the veins of the heart (p p. 281 and 316).
2. The superior vena cava (p. 316). Find its branches. Trace the azygos as far back as the diaphragm without dissecting it at all.
Find the division of the innominate (p. 318) into subclavian and external jugular.
Trace next—
3. The external jugular (p. 319). Remove the skin from the side of the face and trace its branches (Fig. 131). The internal jugular, vena facialis profunda, the submentalis, and the deep terminal branches of the posterior facial cannot be followed at this time; veins shown on Fig. 131 should all be found, however.
4. Trace the thyrocervical axis and its branches (p. 293). Add them to your diagram of the subclavian.
5. Follow the common carotid artery (p. 283) and internal jugular vein (p. 320). Find the division of the common carotid into its terminal branches and then dissect its lateral branches and those of the internal jugular (see Fig. 119).
6. The external carotid (p. 285, and Fig. 119). Follow its branches with the exception of the internal maxillary.
7. The internal maxillary (p. 287). Find its inferior alveolar branch first and follow it by cutting away with bone-forceps the ventral border of the lower jaw. To follow its other branches and those of the carotid plexus, remove the zygomatic arch, cut the temporal, masseter, and pterygoid muscles, and cut the mandible behind the incisor teeth and remove it. The branches which pass into the skull are not to be followed at present. The posterior facial vein (p. 323), the vena facialis profunda (p. 323), and the submental vein (p. 323) may be followed at the same time.
8. The internal carotid (p. 285). Follow it to the point where it enters the cranium.
9. Trace the other branches of the costocervical axis (p. 292). To do this, cut the arteries and nerves of the axilla on the side on which they have been dissected, allowing the arm to fall backward. Then trace the branches of the costocervical axis with tracer, scalpel, and bone-forceps, taking care not to injure the vertebral artery. Add these branches to your diagram of the subclavian.
10. The vertebral artery (p. 291). Trace it to the foramen transversarium of the sixth cervical vertebra. Then with bone-forceps follow it to the atlas and into the atlantal foramen. Add this to your diagram of the subclavian.
11. The basilar artery (p. 291) and the other arteries of the brain (p. 289) are best studied on a preparation, similar to that shown in Fig. 121. To obtain such a preparation it is only necessary to remove the brain (for directions, see p. 462) of a specimen in which the arteries have been injected.
(12. Veins of the brain and dura mater (p. 324). These can be worked out only with much difficulty, except on specimens injected with gelatine. The skull must be chipped away and the veins followed without destroying them.)
13. Trace the pulmonary veins (p. 315) (filled with red injection) and the pulmonary artery (p. 280).
IV. Vessels in the abdominal cavity.
1. Open the abdominal cavity; find the superior mesenteric vein (p. 326, and Fig. 132) in the duodenal mesentery near the border of the pancreas. Inject this in both directions with white starch and then dissect the portal vein and its tributaries without injuring any of the structures in the abdomen (p. 326, and Fig. 132).
2. Follow the inferior vena cava (p. 325) from the heart to the diaphragm and then follow it to its tributaries in the abdominal cavity.
3. Dissect the branches of the abdominal aorta (p. 301) and of the inferior vena cava (Fig. 126). Make diagrams of the vessels dissected and review as far as necessary the viscera concerned.
V. The external iliac and its branches (vessels of the hind limbs) (pp. 309 and 329, and Figs. 127, 128, and 163).
Follow the branches of the external iliac arteries and the corresponding veins in the same manner as the vessels of the arm were traced, cutting the muscles only so far as absolutely necessary. Make diagrams of the vessels dissected.
Make a diagram (a) of the arterial system as a whole; (b) of the venous system as a whole.