Thorax

The thorax is a part of animal anatomy. It refers to the region of the core body just below the head or neck and above the abdomen. While most vertebrates—animals with backbones and skeletal structures—and certain arthropods—such insects—have distinct thoraxes, the structure, organs, and exact location differ dramatically among the various types of animals.

In humans and most mammals, the thorax is one of the most critical portions of the body. It covers the chest area, which contains the heart, lungs, ribs, breastbone, and the thoracic portion of the spine. Because of that, the thorax is a site that is vulnerable to many diseases and adverse conditions. Insect bodies are divided so that there are clear separations between the thorax, head, and abdomen. Arachnids developed a body structure known as the cephalothorax, which consists of a head and thorax fused together.

Background

The term thorax is Greek in origin. It initially referred to armor designed to protect the chest, such as a breastplate. While it is often used as a synonym for chest, the term thorax applies to the upper back, sides, and all the internal organs and structures that they surround. The bottom of the neck marks the uppermost point of the thorax, while the lowermost point is at the top of the belly. Humans’ lower torso is called the abdomen, mostly containing digestive organs.

The spine, or vertebral column, runs through the back of the thorax. It helps give structure and form to the body. It also protects the spinal cord, a key part of the body’s nervous system. Through it, nerves reach across the body, helping humans to control the body consciously and reflexively react to stimulation.

In humans, the ribcage is a series of bones that help define the chest, with a breastbone, or sternum, connecting them in the front center. These bones also help protect some of the body’s most precious organs. The lungs and the heart rest within the ribcage. Cartilage gives the ribcage some flexibility to account for muscle movement and the expanding and contracting of the lungs. The arms, or front legs in other organisms, connect to the pectoral girdle, which attaches to the thorax. While the arms contain muscles, some muscles within the thorax help with arm movement as well.

Lungs gather air that mammals breathe in through their mouths and noses. They sort oxygen from the inhaled air and transfer it to the body’s bloodstream. The heart pumps blood throughout the body. Oxygen-rich blood moves through the body, nourishing various organs and tissues. Oxygen-poor blood is pumped back to the lungs, where waste is expelled, and the blood is replenished with fresh oxygen. The diaphragm forms the internal barrier between the thorax and abdomen. It also helps regulate the body’s breathing.

Human thoraxes are vulnerable to many ailments. While the skeletal structure protects the heart and lungs from most external trauma, the organs can be damaged by disease or environmental contaminants. Because of the vital nature of these organs, damage or disease can be devastating.

Overview

Thoraxes differ among species. For instance, snakes have no distinction between the thorax and the abdomen. Bird thoraxes are similar to mammals’, but they are more compact, providing birds with superior balance and positioning their wings at the best point in the body to enable flight. Birds use a different method from mammals for breathing, with lungs that can hold a fixed amount of air, rather than expanding and contracting. They lack diaphragms altogether and use the motion of their bodies sternum and ribs to expand air sacs to draw in the air they need.

Insect thoraxes are separated from their heads by a membrane called the cervix. They contain three primary sections: the prothorax, mesothorax, and metathorax. Insect thoraxes’ primary function is locomotion. All insect legs and wings join the body at this section. Most flying insects have specially designed thoraxes that allow for asynchronous muscle movement. This allows them to make extremely rapid wing beats and immediate adjustments to speed and direction. Each of an insect’s three pairs of legs is associated with one of its thorax sections. Most winged insects have two pairs of wings, with one attached to the mesothorax and another to the metathorax.

Unlike mammals, insect thoraxes lack lungs and hearts. Insects obtain oxygen primarily through a network of tube-like structures connected to spiracles located along their thorax and abdomen. They have an open circulatory system, which surrounds organs and tissues with blood, rather than delivering it through blood vessels. They still have dorsal vessel (heart) divided into multiple chambers to circulate the blood through the body. However, the hearts are located in insects’ abdomens. Insects have no internal skeletal structure. Instead, they are encased in a hard outer shell known as the exoskeleton. This helps an insect body retain its form. When insects grow, they do so by molting, shedding their old exoskeleton and forming a new one to cover their increased size.

Arachnids also possess exoskeletons. The cephalothorax controls their limbs as well, with all eight connecting to it. In addition to legs, arachnids have pedipalps, which function as a combination of mouthparts and limbs. They connect to the cephalothorax and can be used to subdue prey. Scorpions have some of the most recognizable pedipalps, taking the form of pincers.

Crustacean thoraxes share many similarities to insects and arachnids. There is a lot of variation among them, with some having distinct segmented thoraxes like insects and others having cephalothoraxes like arachnids. Still others have a greatly reduced or unsegmented abdomen extending from the thorax.

Bibliography

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