The intercostal muscles

How they differ along the set

What changes from one end of the series to the other, which is most of what there is to know about any one of them.

• The spaces between the ribs are filled by the intercostal muscles, which lie in two layers, external and internal, and cross each other.Miller 2020, p. 523
• The external layer is the thicker of the two, four or five millimetres in a large dog, and thins over the floating ribs.Miller 2020, p. 523
• Its fibres leave the caudal border of one rib and run down and back to the cranial border of the next.Miller 2020, p. 523
• The external layer runs from the levator muscles of the ribs down to the costochondral junctions, and may reach into the spaces between the cartilages.Miller 2020, p. 523
• It is missing from the first two or three interchondral spaces, because there it ends at or above the costochondral junction.Miller 2020, p. 523
• With each space further back the external layer reaches further down the rib.Miller 2020, p. 523
• The internal layer is the thinner one, two or three millimetres in a large dog.Miller 2020, p. 525
• It reaches from the vertebral column, leaving only a small triangle free beside the vertebrae, to the lower ends of the ribs and their cartilages.Miller 2020, p. 525
• Its fibres run the other way, up and forward from the cranial border of one rib to the caudal border of the one in front, and they stand steeper than the fibres outside them.Miller 2020, p. 525
• The angle those fibres make falls from about 78 degrees near the vertebral column to about 54 degrees near the sternum.Miller 2020, p. 525
• Twelve levator muscles of the ribs are special formations of the external layer, from the transverse processes of the first to the twelfth thoracic vertebra to the cranial border of the next rib behind.Miller 2020, p. 525

Named parts

What a member of this set carries, and what each part is called in the Nomina Anatomica Veterinaria.

External intercostal musclesMusculi intercostales externi

The thicker outer layer of each space, its fibres running down and back from one rib to the next.Miller 2020, p. 523

Internal intercostal musclesMusculi intercostales interni

The thinner inner layer, its fibres crossing the outer ones and standing steeper.Miller 2020, p. 525

Levator muscles of the ribsMusculi levatores costarum

Twelve spindle-shaped slips, special formations of the outer layer, from a thoracic transverse process to the rib behind.Miller 2020, p. 525

The members

In order along the set. Any one of them can be singled out in the model above, with the picker beneath it.

• Intercostal muscles of the first spaceMusculi intercostales I
• Intercostal muscles of the second spaceMusculi intercostales II
• Intercostal muscles of the third spaceMusculi intercostales III
• Intercostal muscles of the fourth spaceMusculi intercostales IV
• Intercostal muscles of the fifth spaceMusculi intercostales V
• Intercostal muscles of the sixth spaceMusculi intercostales VI
• Intercostal muscles of the seventh spaceMusculi intercostales VII
• Intercostal muscles of the eighth spaceMusculi intercostales VIII
• Intercostal muscles of the ninth spaceMusculi intercostales IX
• Intercostal muscles of the tenth spaceMusculi intercostales X
• Intercostal muscles of the eleventh spaceMusculi intercostales XI
• Intercostal muscles of the twelfth spaceMusculi intercostales XII

References

Where to read this set in full, and where everything above was read.

• NAV 2017Myologia
• Miller's Anatomy of the Dog, 5th ed.The Muscular System: muscles of the thoracic wall
• Dyce, Sack & WensingThe thorax of the dog and cat