2.4.1. Innermost Intercostal Muscles: Transversus Thoracis
Overview of the Innermost Intercostal Muscles
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The remaining muscles of the thorax are described in different ways in different textbooks, so here I have tried to simplify the learning process by categorising them all as 'innermost intercostal muscles'. I think it's really suitable to group them in this way because they all lie deeper than the external and internal intercostal muscles. It's important to note that there are three sets of 'innermost intercostal muscles', and they don't form one continuous muscle layer from front to back. However, the position of each muscle set is distinctive within the ribcage - let's have a look at the first set - transversus thoracis.
Overview of Transversus Thoracis
The transversus thoracis muscle is bilateral (present on both sides of the ribcage) and, out of the three sets of innermost intercostal muscles, it lies most anteriorly in the ribcage. In fact, the fibres fan out from the posterior/internal aspect of the sternum and they insert into the posterior/internal aspects of the 3rd - 6th ribs.
Origin: posterior/internal aspect of the sternal body.
Insertion: posterior/internal aspect of the 3rd - 6th ribs.
Fibre direction: mostly superolateral, although a few extend in an inferolateral direction.
Nerves: intercostal nerves.
Action: assist the external and internal intercostal muscles in respiration, by pulling on the ribs or sternum. The muscles also run deep to the internal thoracic artery and vein - the muscle keeps these vessels tightly in their position, just posterior to the costal cartilages (see Section 4.4.2. for more on this).