The Thorax
2.6 Muscles of the Thoracic Wall
Goal: Identify some of the muscles that cover the thoracic wall (ribs) from superfi- cial to deep. With the exception of inter- costal muscles, the muscles of the thoracic wall have either their origin or insertion in the thoracic region.
The muscles of the thoracic wall are best studied when the thoracic limb is removed.
They include the thoracic part of the cutaneus trunci, latissimus dorsi, scale- nus (dorsalis, medius, and ventralis), tho- racic part of the trapezius, thoracic part of the rhomboideus, serratus ventralis (cervical and thoracic parts), serratus dorsalis (cranial and caudal parts), cra- nial part (origin) of the external and inter- nal abdominal oblique, rectus thoracis,
longissimus thoracis, spinalis et semispi- nalis thoracis, iliocostalis thoracis, exter- nal and internal intercostal muscles.
Identify select muscles of this group as listed on your laboratory ID list and shown on Figures 2.4–2.7. It is not necessary to dis- sect all of the aforementioned muscles. Ask your instructor for advice on which muscles of thoracic wall you should identify.
If not already done, skin the thorax as far caudally as the cranial border of the hind limb (Figure 2.5). Remove the fat and fascia to expose the muscles and define their borders.
With the help of Figures 2.4–2.7, identify the muscles of the thoracic wall listed on your ID list or as suggested by your instructor.
Your instructor may ask you to preserve the cutaneous muscles (Figure 2.4). To save time you may opt to remove the cutaneous mus- cles with the skin. The more time you spend cleaning fascia and fat, the more likely you will end up with a neat dissection. However, keep in mind that some of the specimens are better than others. Identify the cutaneus trunci and cutaneus omobrachialis mus- cles (Figure 2.4).
Next, remove the cutaneous muscles and, with the help of Figure 2.5, identify the latis- simus dorsi, thoracic part of the serratus ventralis, origin of the external abdominal oblique, and the deep pectoral muscles. Note the interdigitation of the thoracic part of the serratus ventralis thoracis with the origin of the external abdominal oblique muscle on the ventral surface of the thorax (Figure 2.5).
Next, remove or reflect the latissimus dorsi muscle dorsally to uncover the dorsal part of the thoracic serratus ventralis muscle (Figure 2.6). To expose the rhomboideus muscle, tran- sect the cervical and thoracic parts of the Box 2.2
In performing lung auscultation in a live animal, you should place your stethoscope in front of the basal border of the lung and caudal to the caudal border of the triceps brachii muscle.
Identify the diaphragmatic line of pleural reflection on the live animal. A needle inserted
cranial to the diaphragmatic line of pleural reflection will typically end up in the pleural cavity. In contrast, a needle inserted caudal to the diaphragmatic line of pleural reflection will typically be in the peritoneal cavity of the abdomen.
trapezius muscle and reflect them dorsally (Figure 2.6).
Transect some of the superficial neck mus- cles as shown in Figure 2.6 to uncover the cer- vical part of the serratus ventralis muscle.
Reflect the serratus dorsalis cranialis dorsally
to identify the epaxial muscles as a group.
(There is no need to identify individual mus- cles; Figure 2.6.)
Remove the thoracic limb by severing sev- eral extrinsic muscles. Abduct the limb and, with the use of a long knife, make a circular
Trapezius m.
(thoracic part)
Cutaneus trunci m.
Cutaneus omobrachialis m.
Latissimus dorsi m.
Ox
Figure 2.4 Superficial muscles of the bovine thoracic wall: left lateral view.
Trapezius m.
(thoracic part)
Latissimus dorsi m.
External abdominal oblique m.
(covered by tunica flava abdominis) Deep pectoral m.
Serratus ventralis m.
(thoracic part)
Ox
Figure 2.5 Lateral thoracic muscles deep to the cutaneous muscles. Dotted lines show the interdigitation of serratus ventralis thoracis muscle with the origin of the external abdominal oblique muscle.
cut starting from the pectoral muscles ventrally in the area between the thoracic limbs. Cut the axillary vessels and branches of the brachial plexus. Keep abducting the limb until the medial surface of the scapula is separated from the serratus ventralis muscle.
Cut the attachment of the dorsal border of the scapula with the trapezius muscle (this may have already been done) and the thoracic and
cervical parts of the rhomboideus muscle and remove the limb. You should end up with a view similar to Figure 2.7.
Identify the long thoracic nerve coursing caudally on the dorsal border of the scalenus dorsalis muscle. More caudally, the nerve courses on the lateral surface of the serratus ventralis muscle. The long thoracic nerve supplies the serratus ventralis muscle.
Serratus ventralis m.
(cervical part) Serratus ventralis m.
(thoracic part) Deep pectoral m.
Rhomboideus m.
(cervical and thoracic parts)
Serratus dorsalis cranialis m.
(aponeurosis)
Epaxial muscles
Ox
Figure 2.6 Select deep muscles of the thoracic wall.
Serratus ventralis
m. (cervical part) Serratus ventralis m.
(thoracic part) Attachment site of serratus ventralis m.
to serrated face of scapula
Long thoracic n.
Brachial plexus
Scalenus dorsalis m.
Scalenus ventralis m.
Deep (ascending) pectoral m.
Brachiocephalicus m.
(reflected dorsally)
Ox
Figure 2.7 Bovine thoracic wall: left lateral view. The left thoracic limb is removed. Note the extent of the thoracic and cervical parts of the serratus ventralis muscle.
Now we are ready to enter the thorax. Remove all of the muscles on the thoracic wall until you uncover the ribs. Care is necessary when remov- ing the intercostal muscles. By making vertical cuts along the cranial and caudal borders of the ribs (watch out for embalming fluid oozing out), being careful not to cut too deep, remove the muscles between all ribs.
Notice the intercostal vessels and nerves (vein, artery, and nerve) running along the caudal border of each rib. At the costochon- dral junctions these neurovascular bundles are present along both the cranial and caudal borders of the ribs. They are more visible inside the thorax deep to the transparent costal pleura.
You may choose one of two methods to expose thoracic viscera.
Method 1: Once you have cleaned the mus- cles of the thoracic wall, use a large nipper, pruning shears, or saw to cut each of the ribs at their vertebral and sternal attachments and along the costal arch but leave the first two ribs in place. These two ribs will be removed later when you are ready to visualize the vessels and nerves in the cranial thoracic region (Figure 2.8).
Method 2: This method is suitable for small and large cadavers laid on the table. It is
described in Appendix A. In this method, reflect the ribs dorsally and cut the diaphragm and abdominal wall ventrally along the dorsal edge of the rectus abdominis muscle. Reflect the ribs and abdominal muscles dorsally as a single flap (see Figure A.2 in Appendix A).
In both methods, use your probe and forceps to perform blunt dissection of intrathoracic structures.
Within the thorax, identify the transversus thoracis muscle fibers coursing on a trans- verse plane over the dorsal aspect of the ster- num. The internal thoracic artery and vein run deep to this muscle. Remember that these vessels are found on the other side of the body. Dorsally, identify the longus colli muscle as it courses from under the body of caudal cervical vertebrae passing through the thoracic inlet to continue along the ventral surface of the bodies of the first few thoracic vertebrae (Figure 2.5).