LAB 9 Thoracic Wall

The heads of the ribs articulate with facets on adjacent vertebral bodies while the tubercle of each rib articulates with an articular facet on a transverse process. For inspiration the chest cavity enlarges by lateral and cranial movement of the ribs which is facilitated by the serratus dorsalis cranialis muscle. However, contraction of the diaphragm is more important for enlarging the chest cavity. Expiration is facilitated by compression of the chest cavity caused by caudal medial movement of the ribs due to the action of the serratus dorsalis caudalis muscle but the external abdominal oblique muscle has a similar and more powerful action on the ribs. Both abdominal oblique muscles compress the abdominal viscera so that the viscera is forced against the diaphragm and thereby the chest cavity is compressed also. This action on the diaphragm has been called the abdominal plunger effect and is the opposite of contraction of the diaphragm. In other words, contraction causes the diaphragm to flatten but the abdominal plunger makes the diaphragm more convex in a cranial direction.

The diaphragmatic line of pleural reflection is the line of separation between the chest cavity and the abdominal cavity. A needle passed through the body wall above and cranial to this line will enter the pleural cavity and this is useful for withdrawal of fluid (thoracentesis) for diagnostic use.

The cranial dorsal thoracic region has several types of humps. Most simple is the withers of the horse which is a subtle hump formed by elongated spinous processes. The supraspinous bursa is interposed between the spine of T2 and the supraspinous ligament. Infection of this bursa results in fistulous withers, a condition that once was common in the U.S. but now is rare for several reasons. However, it is more common abroad (image 9-14). Zebu cattle (Bos indicis) have a hump that is formed by the rhomboideus muscle but the hump of American Bison is formed by elongated thoracic spines. Camel humps are formed by firm fibrous fatty tissue somewhat similar to the nuchal crest of the horse.