Contraction of the internal intercostal muscles decreases the anteroposterior diameter of the thorax, aiding in expiration. The abdominal muscles aid in forceful expiration (such as coughing) by increasing intraabdominal pressure; the increased pressure is transmitted through the diaphragm to the pleural space.

Explanation:

The contraction of the internal intercostal muscles in man causes forced expiration.

Internal Intercostal Muscles:
The intercostal muscles are the muscles located between the ribs in the thoracic cavity. There are two types of intercostal muscles: external intercostal muscles and internal intercostal muscles. The internal intercostal muscles are located deep to the external intercostal muscles.

Function of Internal Intercostal Muscles:
The primary function of the internal intercostal muscles is to assist in forced expiration. During normal respiration, the external intercostal muscles contract to help expand the thoracic cavity and facilitate inspiration. However, during forced expiration, the internal intercostal muscles contract to pull the ribs downward and inward. This action decreases the size of the thoracic cavity, increasing the pressure within the lungs, and forcing air out.

Forced Expiration:
Forced expiration is a more active process than normal respiration. It occurs during activities that require increased effort or during certain respiratory conditions. During forced expiration, the internal intercostal muscles contract vigorously, along with other muscles such as the abdominal muscles, to actively push air out of the lungs. This helps to empty the lungs more completely and quickly.

Other Options:
a) Inspiration: Inspiration is the process of inhaling air into the lungs. The contraction of the external intercostal muscles and the diaphragm is primarily responsible for inspiration, not the internal intercostal muscles.
b) Normal Respiration: Normal respiration refers to the regular breathing process that occurs at rest. The primary muscles involved in normal respiration are the diaphragm and the external intercostal muscles.
c) Aspiration: Aspiration refers to the inhalation of foreign material into the respiratory tract, such as food or liquids. The contraction of the internal intercostal muscles is not directly related to aspiration.