Sternum

STERNUM OR BREAST - BONE
(Ox, Sheep and Goat, Horse, Pig, Dog, Rabbit, Fowl)

Ox

The sternum is a medially placed-segmented bone, forming the floor of the thorax.It is made up of the seven bony segments,sternebrae and is elongated from before backwards.The segments are connected by intervening cartilages. It is compressed laterally till about its middle and thereafter it is flattened from above downwards.It is directed obliquely downwards and backwards.  

  • It presents two surfaces, two borders and two extremities.
  • The dorsal face is narrow in front but becomes gradually wider behind. This face gives attachment to the internal ligament in the middle and transverses thoracis muscle laterally.It forms the floor of the thorax.
  • The ventral face is extensive and is strongly convex in front but more or less concave behind the second sternebrae.The surface affords attachments to the pectoral muscles and the recti abdominis muscles. The two surfaces are marked by transverse lines indicating the lines of fusion of the sternebrae.
  • The lateral borders are thick in front but become thinner gradually behind.
  • Each border presents eight articular cavities for articulation with the ventral extremities of the first eight costal cartilages.The first of these is on the anterior part of the dorso-lateral aspect of the first sternebrae. Excepting the first and the last two, the other cavities are between the sternebrae. The facets for the seventh and eighth costal cartilages often become confluent.
  • The anterior extremity is the anterior end of the first sternebrae, which forms the manubrium sterni or presternum.The posterior end of this segment articulates with the anterior end of the second sternebrae to form a diarthrodial joint.The anterior extremity furnishes attachment to the sternocephalicus and sternothyro-hyoideus muscle.
  • The posterior extremity or metasternum is formed by the xiphoid cartilage, which is thin, flexible and nearly circular. Between the pre and metasternum is the body or mesosternum.
  • The diaphragm is attached across the upper concave face of xiphoid cartilage close to its junction with the last sternebrae.The part of the cartilage behind the line of attachment of the diaphragm forms the floor of the abdomen.The ventral face furnishes attachment to the transverse abdominis and linea alba.

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Sheep and Goat

  • Resembles in general that of ox.
  • The number of segments may be reduced to six.
  • The first segment is cylindrical with enlarged ends. The second and third are wide and flat.
  • The last is long and narrow.

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Horse

  • It is composed of seven sternebrae.
  • It is canoe or boat shaped and presents three surfaces, three borders and two extremities.
  • The superior surface is narrow and triangular.
  • The two lateral surfaces are convex above, slightly concave below and present costal cavities on their upper parts.
  • The dorsolateral borders separate the dorsal and lateral surfaces.
  • The inferior border is the keel.
  • The anterior extremity presents a laterally compressed cariniform cartilage, which presents a deep notch superiorly for articulation with the two first costal cartilages
  • The first sternebrae is fused with the second
  • The manubrium is made up of the cariniform cartilage and the first sternebrae.

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Pig

  • The sternum consists of six segments and resembles that of Ox in general.
  • The first segment is long and bears a blunt pointed cartilage at its anterior end, its posterior end forms a diarthrodial joint with the body.
  • The last segment has a long, narrow part, which bears the xiphoid cartilage.

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Dog

  • The sternum is composed of eight sternebrae - all fused.
  • Each sternebrae slightly compressed from side and constricted in its middle
  • The anterior end bears a small presternal cartilage.
  • The xiphoid cartilage is narrow.

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Rabbit

  • There are 6 sternebrae.
  • Body of the sternum extends from 2 nd to 5 th sternebra.
  • Xiphisternum is the last sternebra formed from the bony xiphoid process and the terminal spatulae - xiphoid cartilage.

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Fowl

  • The sternum differs from the corresponding structure of domestic mammals since it is formed entirely of bone in the adult. It results from the fusion of five pieces, which have separate centres of ossification in the chick.
  • The central wide part forms the body of the bone. In front of this is the anterior process -the rostrum or manubrium and behind is the posterior process or metasternum.
  • The dorsal surface of the body is concave and pierced by foramina, which permit air to enter the interior from the adjacent air sac. The ventral surface is encroached upon by the lower borders of anterior and posterior processes.
  • The anterior process or rostrum is short and on either side of its root is an elongated facet for articulation with the coracoid bone.
  • The posterior process or metasternum is very long being in fact more extensive than the body itself. It carries ventrally, a thin plate of bone called the keel or sternal crest, which affords attachment to the pectoral muscles.
  • The keel is broad in front and fades out posteriorly. United to each lateral border of the body are the antero-lateral or costal process in front and the postero lateral process behind. The costal process projects upwards and forward. The postero-lateral process is short and divides into a medial and lateral xiphoid processes. The lateral division is broad, plate-like and covers the l ast two or three ribs. The medial division is longer and projects backward parallel to the lateral border of the metasternum. The deep notches between them and between the posterior division and metasternum are closed in life by tough membrane. Between the antero-lateral and postero-lateral process, the lateral border of the body shows four depressions for articulation with the third, fourth, fifth and sixth ribs. Occasionally the last rib also meets the sternum.

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Last modified: Friday, 13 April 2012, 9:28 AM