Part 2: External Thorax

Continue to work on the side of the animal that has the dissected forelimb and re-identify the large, fan-shaped serratus ventralis m. which has two parts: cervicis and thoracis; these extend toward the neck and thorax respectively. Identify both parts of the serratus ventralis m. (See DG Fig. 2-76, A) (This muscle was previously identified in Chapter 1.)

On that same side of the animal, identify the scalenus m. on the lateral aspect of the neck and thoracic wall, just ventral to the serratus ventralis m. The scalenus usually appears to be divided into three slips of muscle attaching to the first few ribs. (See DG Figs. 2-76, A & B, and 2-82, A)

• Dissection Note: You will also see a small muscle called the rectus thoracis extending from the scalenus down toward the ventral midline, but it is not significant for our dissection purposes. (See DG Fig. 2-76)

On the side of the animal with the dissected forelimb, transect the rhomboideus m. (all three parts) ~1cm from the dorsal border of the scapula and reflect the majority of the muscle toward the dorsal midline. Reflect the forelimb (and attached serratus ventralis m.) ventrally.

If not already completed, detach the latissimus dorsi m. from the ribs and reflect it dorsally to the mid-dorsal line of the back.

On that same side of the animal, identify the intercostal mm. found in the intercostal spaces between each consecutive rib. You should note that there are two types of intercostal mm., external and internal. (See DG Figs. 2-76, B & C, 3-5 and 3-6)

•  External intercostal mm. run caudoventrally from the caudal border of one rib to the cranial border of the next rib in line. Internal intercostal mm. run the opposite direction, running cranioventrally from the cranial border of one rib to the caudal border of the rib just cranial to it.

• Transect and reflect a ‘flap’ of the external intercostal mm. in one or two of the intercostal spaces. Observe the underlying internal intercostal mm. 

  • • Dissection Note: These muscles are thin and delicate, so try to make a very superficial incision in the external intercostal mm. to create the flap.

Identify the thoracolumbar fascia which is a thick, glistening sheet of deep fascia covering the epaxial muscles of the thorax and lumbar regions.

Dorsal to the serratus ventralis m., on the side of the animal with the dissected forelimb, identify the serratus dorsalis m. which also has two parts: cranialis and caudalis. (Note that in the cat, the serratus dorsalis m. (especially the caudalis part) is more extensive than what is seen in the dog.) (See DG Figs. 2-76, A, B and 2-81)

• Identify the serratus dorsalis cranialis m. which arises via an aponeurosis from the thoracolumbar fascia (deep to the rhomboideus m.). It runs caudoventrally to insert on the ribs. (Note the ‘serrations’ as it inserts on the ribs.) (See DG Fig. 2-76, A, and 2-81)

  • • Transect this muscle through its aponeurosis and reflect the main muscle mass ventrally.

• Caudal to the serratus dorsalis cranialis m., identify the serratus dorsalis caudalis m. which is smaller and less distinct than the cranialis portion. It arises from the thoracolumbar fascia as well and runs cranioventrally to insert on the caudal aspect of the last three ribs. (Typically you will see 3-4 serrations/slips of muscle in the caudalis portion.) (See DG Fig. 2-76, B)

  • • Transect this muscle through its aponeurosis and, if needed, reflect the main muscle mass ventrally.

Now we will move on to the dissection of the epaxial mm. Recall that the epaxial muscles generally lie dorsal to the transverse processes of the vertebrae and are associated with the vertebral column and ribs. Here we will dissect some of these muscles that are divided into three parallel muscle systems running longitudinally: iliocostalis system, longissimus system, and transversospinalis system. (See DG Fig. 2-81)

• Dissection Note: Continue the dissection of the entire epaxial region on the side of the animal with the dissected forelimb.

The most lateral of the three epaxial systems is the Iliocostalis System; identify muscles of the Iliocostalis System. This system extends from the ilium to the seventh cervical vertebra. (See DG Figs. 2-81 and 2-82, A & B)

• Transect and reflect the thoracolumbar fascia and remove excess fascia and fat to expose the underlying epaxial muscles and identify the Iliocostalis System.

• Comment: The Iliocostalis System has two parts, the iliocostalis lumborum and iliocostalis thoracis, but you do not need to differentiate them.

  • • The iliocostalis lumborum m. is seen arising from the ilium and inserting on the transverse processes of the lumbar vertebrae and the last few ribs. (Note that iliocostalis lumborum fuses to longissimus lumborum in the lumbar region.) (See DG Fig. 2-82, B)

    • The iliocostalis thoracis m., which is long and narrow, extends from the twelfth rib to the transverse process of the seventh cervical vertebra. (Note the slips of muscle that extend between, and overlap, the ribs.) (See DG Fig. 2-82, B)

• Dissection Note: Be sure you have completed the transection and reflection of the serratus dorsalis m. (cranialis and caudalis parts) before attempting to complete the dissection of the epaxial muscles!

Moving just medial to the Iliocostalis System, identify muscles of the Longissimus System. This system extends from the ilium to the head. (See DG Figs. 2-76, A & B, 2-81 and 2-82, A & B)

• Comment: The Longissimus System consists of three parts: thoracis et lumborum, cervicis, and capitis, but you do not need to differentiate them.

  • • The longissimus thoracis et lumborum can be seen extending from the ilium to the seventh cervical vertebra with insertions on the ribs within the thoracic portion of the muscle. (Recall that in the lumbar region the longissimus and iliocostalis mm. fuse and can be hard to differentiate.) (See DG Fig 2-82, B)

    • The longissimus cervicis is the cranial continuation of the longissimus muscle into the neck region. This portion typically has four fascicles inserting on the transverse processes of the last few cervical vertebrae. (See DG Fig 2-82, A & B)

    • The longissimus capitis is a distinctive muscle traveling toward the head. It is seen extending from the first three thoracic vertebrae to the mastoid part of the temporal bone of the skull. (See DG Fig 2-82, A & B)

• Dissection Note: Note that the splenius m. (a muscle from the Transversospinalis system, the next system you will identify) may obscure your view of the more cranial parts of the longissimus system.

The most medial of the three epaxial systems is the Transversospinalis System; identify muscles of the Transversospinalis system (which has numerous parts). This system is complex and contains a number of different groups of muscles extending from the sacrum to the head. We will dissect only a few of these muscles in the neck region, the splenius and the semispinalis capitis. (See DG Figs. 2-76, A, 2-81 and 2-82, A & B) (Note that in some of the images you will see terms like “spinalis, “spinalis thoracis” or “spinalis et semispinalis,” etc. – these are all parts of the Transversospinalis System but we will not be dissecting them.)

• In the neck region, on the dorsolateral side, identify the splenius m. The splenius is a large, flattened muscle just deep to the rhomboideus capitis m.

  • • Dissection Note: In the cat, the splenius and longissimus capitis mm. may be fused/difficult to separate as you move cranially. (See DG Fig. 2-81)

    • Transect the splenius m. ~2cm caudal to its insertion and reflect the main part of the muscle dorsally. (See DG Fig. 2-82, A)

• Deep to the splenius m., identify the semispinalis capitis m. and differentiate its two parts: biventer cervicis m. and complexus m. (The biventer cervicis is dorsal to the complexus.) (See DG Fig. 2-82, A & B)

  • • In the dog, locate the fascial seam between the biventer cervicis and complexus mm. and separate them to view the nuchal ligament (dog only; no nuchal ligament present in the cat). The nuchal ligament is a (paired) yellow elastic band that extends from the spinous process of the first thoracic vertebra to the spine of the axis (C2). (See DG Fig. 2-84)

    • Dissection Note: In the cat, the biventer cervicis and complexus mm. are generally difficult to separate and you need not do so for this dissection (since the cat lacks a nuchal ligament, hence no need to look for one).

    • Comment: On the ligamentous skeletal model in the demonstration area, you should also note the supraspinous ligament in the dog. The supraspinous ligament continues the nuchal ligament caudally and extends from the spinous process of T1 to the caudal vertebrae, passing from one spinous process to another. (Note that cats lack the supraspinous ligament.) (See DG Fig. 2-84)

Two palpable structures associated with the thorax/rib cage that you should note are the thoracic inlet and the costal arch.