Final Answer:
The intercostal muscle with fibers running in an inferoanterior direction, described as "hands in pockets," is the External intercostal muscle. It aids in the elevation of the ribcage during inhalation. The answer is a) External intercostal muscle. It aids in the elevation of the ribcage during inhalation.
Step-by-step explanation:
The intercostal muscles are essential for respiratory movements, playing a crucial role in the expansion and contraction of the ribcage during breathing. In the context of the direction of muscle fibers, the muscle that runs in an inferoanterior direction, often described as "hands in pockets," is the External intercostal muscle. These muscles are positioned externally between the ribs and are involved in the elevation of the ribcage during inhalation. Their orientation aids in the expansion of the thoracic cavity, facilitating the intake of air into the lungs. In contrast, the internal intercostal muscles (b) have fibers that run in a superoanterior direction and are involved in forced expiration, while the innermost intercostal muscles (c) are closely associated with the internal intercostals. Transversus thoracis (d) is located on the anterior thoracic wall but is not considered part of the intercostal muscles.