Chapter 51, Problem 1PL

The size of the thoracic cavity is increased by contractions of all of the following muscles except the
a. diaphragm.
b. external intercostals.
c. pectoralis minor.
d. external oblique.

Summary Introduction

Introduction:

Breathing is the movement of air from outside the body through the bronchial tree and into the alveoli and the reversal of this air movement allow gas exchange between air and blood. The cause for these movements is the change in the size of the thoracic cavity that result from skeletal muscle contractions.

Answer to Problem 1PL

Correct answer:

The correct answer is option (d) external oblique.

Explanation of Solution

Explanation for the correct answer:

Option (d)external oblique. The external oblique aids expiration by compressing abdominal contents which helps to raise the diaphragm. The external oblique originates at the anterior surface of ribs five to twelve. So, the correct answer is option (d).

Explanation for the incorrect answers:

Option (a) diaphragm. The diaphragm originates at the costal cartilages and ribs seven to twelve. Its insertion occurs at the central tendon of diaphragm.The diaphragm contraction flattens itself and thus, it expands the size of thoracic cavity. Therefore, option (a) in incorrect.

Option (b) external intercostals. External intercostal muscles originate at the inferior border of superior rib. These muscles tend to elevate ribs which expands the thoracic cavity during inspiration. Therefore, this option is incorrect.

Option (c) pectoralis minor. The pectoralis minor originates at the sternal ends of ribs three to five. This muscle elevates the 3^rd and 5^th ribs which aids in increasing the size of thoracic cavity. Therefore, this option is incorrect.

Conclusion

The correct answer is Option (d) external oblique because the external oblique aids expiration by compressing abdominal contents which helps to raise the diaphragm.