Thorax (middle of the chest


The thorax is a division of an animal's body that lies between the head and the abdomen. Basically, its the middle of the human chest.

The primary function of the thorax is respiration. [1] The ribs and the diaphragm move so that the thoracic cavity increases and decreases in size during the inspiratory and expiratory phases of respiration. It also probably aids in returning venous blood back to the heart because of the negative pressure produced with respiratory movements. Secondarily it serves to protect the organs located within its cavity plus some organs of the abdominal cavity. The main thoracic organs which you will examine in your dissection of the thorax are the:

  • lungs
  • heart

The other structures are:

  • aorta and its branches
  • superior and inferior vena cava
  • trachea and primary bronchi
  • sympathetic trunks and their associations
  • azygos and hemiazygos venous systems

The first rib[]

it is atypical. It is found to be short, flat and more sharply curved than any of the others. It has upper and lower surfaces, with outer and inner borders, and on its head there is one articular facet only.

The upper surface has two grooves for the subclavian artery and subclavian vein, separated by the scalene tubercle for the attachment of the scalene anterior muscle.

This rib has very little movement during respiration and serves as a base attachment for the intercostal muscles and the ribs below. In other words, during respiration, the muscles in the first intercostal space contract, drawing up on the rib below, which in turn allows its muscles to pull up on the rib below it and so forth, until all ribs have moved through a small distance. The combined movements increase the transverse and anteroposterior diameters of the thoracic cavity. [2]

Thoracic Vertebrae[]

There are 12 thoracic vertebrae. The 1st and 12th are called atypical and the rest are typical. All of the typical vertebrae have the same characteristics. The 1st and 12th vertebrae have slightly different characteristics than the typical ones.[3]

Muscles of the thorax[]

The muscles of the thorax consist of the intercostals and diaphragm. The intercostal muscles are arranged as three layers (external layer, internal layer and an incomplete innermost layer) between the ribs. The diaphragm closes the thoracic outlet and separates the thoracic cavity from the abdominal cavity. The three layers of the intercostal muscles are:

  • external layer -- external intercostal
  • internal layer -- internal intercostal
  • innermost layer -- transversus thoracic (anterior), innermost (lateral) and subcostal (posterior)

The diaphragm is the most important muscle of the thoracic wall. During normal respiration, this muscle is the primary component.

Nerves of the Thoracic Wall[]

The thoracic wall is supplied by the intercostal nerves which are the anterior primary rami of spinal nerves. A typical spinal nerve is shown in the adjacent diagram.

  • spinal cord
  • dorsal (sensory, afferent) root
  • ventral (motor, efferent) root
  • spinal nerve
  • dorsal primary ramus (mixed)
  • ventral primary ramus (mixed)
  • white communicating ramus
  • gray communicating ramus
  • sympathetic ganglion

Once you have the names down, you should try to understand what is going on in each of the branches.

Starting at the spinal cord, we can work our way out to the periphery describing how a spinal nerve is formed and how it branches.

When the spinal cord is examined in cross section you can usually pick out two colors on its surface: white and gray. This is called the white matter and the gray matter of the spinal cord. The gray matter forms a butterfly-like image with dorsal horns and ventral horns.

The white matter is made up mostly of nerve fibers running up and down the spinal cord. The gray matter is made up mainly of cell bodies of nerve cells, this giving a grayer appearance.


Respiration is the process of exchanging O2 with CO2. In order to get the oxygen into the lungs, all of the structures that you have just learned act together to increase the area of the thoracic cavity. During inspiration, the lateral dimensions of the thoracic cavity are increased by the 7-10th ribs moving laterally (similar to bucket handles). The anteroposterior dimension is increased by the sternum being pushed forward by the true ribs (1-6). The superoinferior dimension is increased by the diaphragm contracting and becoming lower. During restful breathing, the diaphragm probably does most of the work, although small movement in all directions probably occur. During increased need for oxygen (exercise, pathology), the lateral and anterioposterior movements will be increased. When the thoracic muscles can no longer do the job, other muscles attaching to the thorax will be called into action (pectoralis major and minor, sternomastoid, etc.) During expiration, the intercostal muscles and the diaphragm relax and the elastic fibers of the lung and the costal cartilages recoil to their original state before inspiration. The automatic nature of the respiratory cycle is controlled in the respiratory centers of the brain stem. [4]


  1. Pheasant, Steven & Schroeder, Jane K., "Geriatric rehabilitation manual: The aging bony thorax", 2nd Ed. 2007
  2. [1] Nadalo, Lennard A, MD. & Jones, Kory, MD. eMedicine: Rib Fractures,, Jan 16, 2009
  3. [2] International Spinal Research Trust Institute, "The spinal column and spinal levels",, 2010
  4. [3] Medical Dictionary, "Respiration",, 2010