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The composition of the chest and how the upper and lower mouths are surrounded.
The thorax is a bony structure composed of thoracic vertebrae, ribs, sternum and intercostal muscles, and the upper and lower openings of the thorax are formed by ribs and intercostal muscles.

1. The upper opening of the thoracic cavity is surrounded by the upper sternum, the upper edges of the first to fourth ribs and the upper edges of the fifth to eighth costal cartilage. The upper part is wide and the lower part is narrow, and the upper part is wider than the lower part. This is because the fifth to eighth costal cartilages meet in sternal angle to form sternal angle, while the sternal angle and the spinous process of the seventh cervical vertebra are on the same horizontal line, which makes the upper mouth wider than the lower mouth in the horizontal direction. At the same time, the angle formed by sternal angle and the ribs on both sides further widened the width of the upper mouth.

2. The lower opening of the chest consists of the ninth to twelfth ribs, rib bow and sword. The shape of the lower mouth is similar to an inverted trapezoid, and the front and rear diameters are slightly larger than the left and right diameters. The rib arch forms an outward convex arc at the lower thoracic opening, which makes the lower opening narrower than the upper opening in the horizontal direction. At the same time, the existence of xiphoid process also makes the lower mouth narrower than the upper mouth vertically.

3. Between the upper and lower openings of the thoracic cavity, ribs and intercostal muscles form intercostal spaces. The intercostal space is an outward convex arc in the side view of the thorax, which makes the thorax also conical in the side view.

The main function of the chest:

1. Protect the organs in the chest cavity: The chest cavity is composed of ribs, sternum and spine, forming a solid framework, which provides protection for the heart, lungs, esophagus and other important organs. The integrity of the chest can prevent external injuries from destroying organs in the chest.

2. Maintain respiratory function: The shape and size of the chest will affect the respiratory movement. In the process of breathing, the chest cavity expands and contracts, driving the lungs to breathe, inhaling oxygen and expelling carbon dioxide. The normal movement of the chest is very important for maintaining the normal breathing function of the human body.

3. Participate in upper limb movements: the thorax forms joints with the upper limb bones such as scapula and humerus, and the movement of the thorax can drive the upper limb to carry out various activities. For example, shoulders, arms, elbows and other movements require the participation of the chest.

4. Maintain body posture: The integrity of the chest is of great significance for maintaining body posture. If the chest is damaged or deformed, it may lead to spine bending and shoulder inequality, which will affect the posture and appearance of the body.