Thorax

The thorax can be defined as the middle section of the human body between the neck and the abdomen. It is occupied by vital organs and structures, which make it functionally crucial in breathing, protection of the vital organs, and circulatory functions. The thorax is loosely referred to as the chest.

1.0Thorax of Human being

Anatomy of the Thorax

1. Thoracic Cavity: The thoracic cavity is bounded by the ribs, sternum, and thoracic vertebrae. It contains:

• Lungs, Heart, Esophagus, Trachea, Bronchi
• Major blood vessels such as aorta, superior vena cava, and pulmonary arteries/veins
• Diaphragm (inferior boundary)

2. Thoracic Wall: The thoracic wall gives structural support and protection to the organs found in the thoracic cavity. It consists of:

• Ribs (12 pairs) and Costal Cartilages
• True Ribs (seven pairs): attached directly to the sternum.
• Ribs 8-10 false ribs attached to the sternum by cartilage.
• Ribs 11-12 floating ribs that are not attached to the sternum.

Sternum (breastbone) consists of three parts.

• Manubrium is the uppermost portion.
• The body is the middle part.
• The xiphoid process is the lowermost, cartilaginous part that ossifies in adults. Thoracic Vertebrae there are 12 vertebrae that forms the posterior boundary of the thorax and articulates with the ribs

3. Diaphragm

• The diaphragm is the dome-shaped muscle that partitions the thoracic cavity to the abdominal cavity. 
• The diaphragm is the main muscle of respiration: upon inspiration, it contracts, whereas during expiration, it relaxes.

Musculature of the Thorax

1. Intercostal Muscles

Intercostal muscles fill the intercostal spaces between the ribs and thus participate in respiration. Intercostals can be classified into the following:

• External Intercostals- Assist in raising of the ribs while inspiring.
• Internal Intercostals- Assist in forced expiration.
• Innermost Intercostals – Deepest layer also involved in rib movement.

2. Accessory Muscles of Respiration

Other muscles involved during difficult breathing:

• Sternocleidomastoid – Elevates the sternum.
• Scalene muscles – Elevate the first two ribs.
• Pectoralis major and minor – Aids in increasing thoracic cavity.
• Serratus anterior – Elevates ribs to help with breathing.

Major Organs in the Thorax

1. Heart

The heart is located centrally within the thoracic cavity, within the mediastinum (the central compartment between the two lungs). The heart is covered by a double-layered protective sac called a pericardium. Major Heart Structures:

• The right and left atrium – blood collection chambers
• The right and left ventricle – pump chambers
• The above are the major vessels:
• • Aorta - primary artery of the body
  • Pulmonary arteries/veins
  • Superior and Inferior Vena Cava - two large veins that return blood into the heart

2. Lungs

The lungs are located:

• The right lung is divided into three lobes (superior, middle, inferior).
• The left lung is divided into two lobes (superior and inferior) and the cardiac notch to accommodate the heart. 
• The lungs have the prime function of gas exchange, acquiring oxygen-depleted blood from the heart through pulmonary arteries, while sending back oxygenated blood back to the heart, through pulmonary veins.

3. Trachea and Bronchi

• The trachea, or windpipe, is a rather wide, flexible tube that runs from the larynx down into the chest, curving to divide and bifurcate into the right and left bronchi, which enter the two lungs. 
• These divide into thinner and thinner bronchioles and finally end in the alveoli, where gas exchange takes place.

Vascular and Nerve Supply

1. Blood Supply

• Aorta-the major artery supplying blood to the body, curves over the heart in the thoracic cavity.
• Pulmonary Arteries – carry deoxygenated blood from the heart to the lungs.
• Pulmonary Veins – return oxygenated blood to the heart from the lungs.
• Intercostal Arteries – supply blood to the intercostal muscles and ribs.
• Internal Thoracic Arteries – branch from the subclavian artery to supply the anterior thoracic wall.

2. Venous Drainage

• Azygos vein system – drains the thoracic wall and upper lumbar region into the superior vena cava.
• Thoracic Veins – drain into the brachiocephalic vein.

3. Innervation

• Phrenic Nerve – the nerve that innervates the diaphragm as well as controls the contraction of its muscle fibers.
• Intercostal Nerves- Their role is to give sensory and motor supply to the wall of the thorax and to the intercostal muscles.
• Vagus Nerve (Cranial Nerve X)- the most important nerve in parasympathetic control of the heart, lungs, and digestive tract

Function of the Thorax

1. Protection

• A protective shield for the vital organs including the heart and lungs, the rib cage forms a solid yet flexible framework through which shock and impacts from the external environment are absorbed by the ribs, sternum, and thoracic vertebrae.

2. Respiration

The thorax allows air in by expanding and then contracting to breathe during respiration. Main parts of respiration:

• Rib expansion during inspiration in which it expands upwards and outwards.
• Contraction of the diaphragm, which increases the vertical space in the thoracic cavity.
• Intercostal muscle activity (assists both inspiration and expiration).

3. Circulation

• The thorax contains the heart and major vessels that deliver blood to the body. 
• Pulmonary circulation occurs within the thoracic space, where oxygenated blood comes to the heart to be directed again to tissues.

Clinical Aspects of the Thorax

1. Thoracic Trauma

• Rib fractures – most common injuries from direct blows or compression.
• Pneumothorax – air enters the pleural cavity, causing lung collapse.
• Hemothorax – blood accumulates in the pleural cavity, often due to trauma.

2. Thoracic Outlet Syndrome

• This results from pressure on blood vessels or nerves between the clavicle and first rib. 
• The patient will experience pains around the shoulder and neck, possible numbness, and certain circulation conditions.

3. Pleural Effusion

• Fluid accumulation in the pleural space, which is sometimes a result of infection, heart failure, or cancer.

4. Cardiopulmonary Resuscitation (CPR)

• During CPR, compressions of the chest are applied on the thorax with the intent of stimulating the heart and conducting circulation.

2.0Thorax of an Insect

• The thorax of an insect, as in other insects, is an entire important body part. 
• It has been specialized for movement and is divided into three parts to support every pair of legs along with some species' wings on either side. 
• The thorax functions as the movement center of the insect and supports attachment points for muscles that help walking, running, and even flying.

General Description

The insect has a body consisting of three general divisions:

• Head
• Thorax
• Abdomen

Thoracic Structure

The insect's thorax is made up of three segments:

• Prothorax
• Mesothorax
• Metathorax

Each thoracic segment supports a pair of legs, and the mesothorax and metathorax support wings in winged species.

1. Prothorax

• This is the first and anterior-most portion of the thorax.
• It does not possess wings but supports the first pair of legs
• It also consists of a pronotum, which is a shield-like structure used for protection of thorax as well as parts of the head.
• Prothorax consists of muscles which will enable controlling of the first pair of legs and on-ground movement.

2. Mesothorax

• The second segment of the thorax is referred to as the mesothorax.
• It supports the second pair of legs and the first pair of wings (forewings), also called tegmina.
• Forewings (Tegmina): These wings are thickened, leathery, and protective, and they are used to cover the hindwings when the insect is at rest. They don't play a very significant role in flight but act as protective covers.
• The muscles in the mesothorax control the movement of the second pair of legs and the tegmina.

3. Metathorax

• It is the last and third part of the thorax.
• The third pair of legs and the second pair of wings (hindwings) are borne on it.
• Hindwings: These are membranous and the foremost for the purpose of flying during the time it flies.
• Muscles inside the metathorax are engaged in both walking and flying. Such muscles here are more specialized in the control of hindwings during the purpose of flying.

Structure and Functioning of the Legs

• Each of the thoracic segments has one pair of legs, which has made the insect a hexapod, that is, six-legged. The various parts of every leg are:
• Coxa – this is the base segment attached to the leg with the thorax.
• Trochanter – it is this small joint that permits movement in the various directions of the leg.
• Femur – it is this large and sturdy segment of the leg, used in locomotion.
• Tibia – it is a long slender segment that provides additional leverage or movement.
• Tarsus – a divided "foot" for walking and gripping the surface. The tarsus ends with claws that can hold it to surfaces.
• These legs are made for speed so insects can sprint across every kind of surface.

Wings - insect are miserable fliers, but they do have wings:

Tegmina / Elytra  (Forwings): These leathery forewings also act as a cover for the hindwings when the insect is resting, to protect the softer, more fragile hindwings.

Hindwings: They are thin membranous wings folded under the tegmina, used for short bursts of flight in cases when the insect has to rush off.

Musculature

• The thorax has large muscles controlling the movement of the legs and wings. The muscles are divided to each of the parts of the thoracic:
• Prothorax Muscles: Controls the first pair of legs.
• Mesothorax Muscles: Controls the second pair of legs and the forewings.
• Metathorax Muscles: Responsible for the third pair of legs and hindwings. Because of that, they contribute to walking and partially to flying.

Thoracic Nervous and Circulatory Systems

1. Thoracic Nervous System

• The thorax has a few ganglia-assemblages of nerve cells responsible for controlling the movements of the legs and wings.
• Each segment of the thorax has its own supply of nerves; thus, it controls accurate movement of the legs and wings.

2. Circulatory System

• Similar to other insects, insects have an open circulatory system that implies hemolymph, or insect blood, passes through the main body cavity without confinement.
• The area of the thorax contains part of the dorsal vessel, a heart by which the hemolymph is pumped into the body and which provides oxygen and nutrients for muscles.

Role Of Thorax In Insect

• Motion: The thorax is adapted for fast, streamlined motion. Insects are among the quickest insects because of their big, strong thigh and knee muscles along with some specialized joints.
• Flight: Although insects do not have particularly good flight, they do use their wings to fly a short distance to run away from predators.
• Protection: Pronotum in prothorax helps to protect such delicate parts of the body-the head and neck.

Thoracic Adaptations Of The Insect

• Speed and Agility: Because of its two pairs of legs and strong thoracic muscles, the insect can run at incredible velocities-one of the fastest speeds in the insect world in terms of proportion.
• Flight: An insect with two pairs of wings, though it does not depend on these strictly to fly for any length of time, means the insect can make a glide or short flights when needed.
• Flexibility: The segmentation of the thorax gives the insect flexibility, which enables it to bend through very narrow gaps and corners as a survival adaptation.