Human rib cage
|Human rib cage|
|The human rib cage. (Source: Gray's Anatomy of the Human Body, 20th ed. 1918.)|
The human rib cage, also known as the thoracic cage, is a bony and cartilaginous structure which surrounds the thoracic cavity and supports the pectoral girdle, forming a core portion of the human skeleton. A typical human rib cage consists of 24 ribs, the sternum (with Xiphoid process), costal cartilages, and the 12 thoracic vertebrae. It, along with the skin and associated fascia and muscles, makes up the thoracic wall and provides attachments for the muscles of the neck, thorax, upper abdomen, and back.
The human rib cage is a component of the human respiratory system. It encloses the thoracic cavity, which contains the lungs. An inhalation is accomplished when the muscular diaphragm, at the floor of the thoracic cavity, contracts and flattens, while contraction of intercostal muscles lift the rib cage up and out.
Expansion of the thoracic cavity is driven in three planes; the vertical, the anteroposterior and the transverse. The vertical plane is extended by the help of the diaphragm contracting and the abdominal muscles relaxing to accommodate the downward pressure that is supplied to the abdominal viscera by the diaphragm contracting. A greater extension can be achieved by the diaphragm itself moving down, rather than simply the domes flattening. The second plane is the anteroposterior and this is expanded by a movement known as the 'pump handle.' The downward sloping nature of the upper ribs are as such because they enable this to occur. When the external intercostal muscles contract and lift the ribs, the upper ribs are able also to push the sternum up and out. This movement increases the anteroposterior diameter of the thoracic cavity, and hence aids breathing further. Finally, you have the transverse. In this situation, it involves mainly the lower ribs (some say it is the 7th-10th ribs in particular) with the diaphragm's central tendon acting as a fixed point. When the diaphragm contracts, the ribs are able to evert and produce what is known as the 'bucket handle' movement, facilitated by gliding at the costovertebral joints. In this way, the transverse diameter is expanded and the lungs can fill.
Breathing may be assisted by other muscles that can raise the ribs, such as sternocleidomastoid, pectoralis major and minor as well as the scalenes. While under most circumstances, individuals respire via eupnea, exercise and other forms of physiological stress can cause the body to require forced expiration, rather than the simple elastic recoil of the thoracic cage, lungs and diaphragm. In this case, muscles are recruited which can help depress the ribs and raise the diaphragm - such as the anterior abdominal wall muscles, excluding the transversus abdominis muscle. Latissimus dorsi can also aid deep, forced expiration.
Another way the thoracic cavity can expand during inhalation is called belly breathing. This also involves a contraction of the diaphragm, but the lower ribs are stabilized so that when the muscle contracts, rather than the central tendon remaining stable and lifting the ribs up, the central tendon moves down, compressing the sub-thoracic cavity and allowing the thoracic cavity and lungs room to expand downward.
These actions produce an increase in volume, and a resulting partial vacuum, or negative pressure, in the thoracic cavity, resulting in atmospheric pressure pushing air into the lungs, inflating them. An exhalation results when the diaphragm and intercostal muscles relax, and elastic recoil of the rib cage and lungs expels the air.
The circumference of the normal adult human rib cage expands by 3 to 5 cm during inhalation.
All ribs are attached in the back to the thoracic vertebrae.
Each rib consists of a head, neck, and a shaft. The head typically has two facets on its surface; one for articulation with the corresponding vertebrae, and one for articulation with the immediately superior vertebrae. 
The upper seven vertebrosternal ribs (costae verae, I-VII). are attached in the head to the sternum by means of costal cartilage. Due to their elasticity they allow movement when inhaling and exhaling.
The 8th, 9th, and 10th ribs are called false ribs (costae spuriae, vertebrochondral ribs, VIII-X), and join with the costal cartilages of the ribs above.The 11th and 12th are also sometimes referred to as false ribs.
The 11th and 12th ribs are known as floating ribs (costae fluitantes, vertebral ribs, XI-XII), as they do not have any anterior connection to the sternum.
Number of ribs
The number of ribs (24; twelve on each side) was noted by the Flemish anatomist Vesalius in his key work of anatomy De humani corporis fabrica in 1543, setting off a wave of controversy, as it was traditionally assumed from the Biblical story of Adam and Eve that men's ribs would number one less than women's. Variations in the number of ribs occur. About 1 in 200-500 people have an additional cervical rib, and there is a female predominance. Intrathoracic supernumerary ribs are extremely rare.
Abnormalities of the rib cage include pectus excavatum ("sunken chest") and pectus carinatum ("pigeon chest"). Bifid or bifurcated ribs, in which the sternal end of the rib is cleaved in two, is a congenital abnormality occurring in about 1.2% of the population. The rib remnant of the 7th cervical vertebra on one or both sides is occasionally replaced by a free extra rib called a cervical rib, which can cause problems in the nerves going to the arm.
Rib removal is the surgical excision of ribs for therapeutic or cosmetic reasons.
Rib cage development & testosterone
In males, expansion of the ribcage is caused by the effects of testosterone during puberty. Thus, males generally have broad shoulders and expanded chests, allowing them to inhale more air to supply their muscles with oxygen.
- Respiratory system examination citing: Health & Physical Assessment, Mosby-Year Book, inc. School of Nursing, Peking University, 2003
- Ribs. http://www1.american.edu/adonahue/k11ribs.htm. [Web]
- "Chapter 19 On the Bones of the Thorax". Retrieved 2007-08-23.
- Kurihara Y; Yakushiji YK, Matsumoto J, Ishikawa T, Hirata K (Jan-Feb 1999). "The Ribs: Anatomic and Radiologic Considerations" (PDF). RadioGraphics (Radiological Society of North America) 19 (1): 105–119. ISSN 1527-1323. PMID 9925395. Retrieved August 13, 2009.
- Kamano H; Ishihama T, Ishihama H, Kubota Y, Tanaka T, Satoh K (June 1, 2006). "Bifid intrathoracic rib: a case report and classification of intrathoracic ribs" (PDF). Internal Medicine (The Japanese Society of Internal Medicine) 45 (9): 627–630. doi:10.2169/internalmedicine.45.1502. PMID 16755094. Retrieved August 13, 2009.
- Testosterone causes expansion of rib cage during puberty as one of secondary sex characteristics.
- Clinically Oriented Anatomy, 4th ed. Keith L. Moore and Robert F. Dalley. pp. 62–64
- Principles of Anatomy Physiology, Tortora GJ and Derrickson B. 11th ED. John Wiley and Sons, 2006. ISBN 0-471-68934-3
- De Humani Corporis Fabrica: online English translation of Vesalius' books on human anatomy.