Let’s dive in to the third rib. As the third rib is a true rib, Clark does not provide a deep description of it. That doesn’t mean there is nothing going on here. The third rib has more mobility than the second and as such has more possible lesion patterns. The most common lesion is that of being upwardly displaced at the vertebral end while being displaced downwards and inwards at the sternal end. This lesion often makes the tubercle more palpable due to the upward displacement of the vertebral end that is perpetuated through ligamentous thickening as the body’s response to stabilize the lesion.
The muscles that attach to the third rib are: pectoralis minor, serratus anterior, serratus posterior superior, iliocostalis cervicis, iliocostalis thoracis, internal intercostal, and external intercostal. On the internal surface of the third rib is attachments to the pleura.
The nerves that are associated with the third rib are: the third thoracic nerve (both anterior and posterior branches), the sympathetic chain ganglion, and the third sympathetic ganglion (with associated influences to the head, neck, lungs, and heart).
The circulatory structures associated with the third rib are: the third intercostal artery and vein (this is the only structure mentioned by Clark, the following structures are ones that I searched for), the axillary artery and vein (externally), the superior intercostal veins (internally), the thoracic duct (internally), as well as the bronchomediastinal trunks (lymphatic vessels that drain the chest wall and mammary tissues). I may have missed some but I will keep digging!
Conditions that are associated with a third rib lesion are pleurisy (due to the anchoring of the pleura and the mechanical strain that is associated with alterations in the position of the rib), angina pectoris/other functional disorders of the heart (via the interrupted connection resulting from pressure on the third thoracic ganglion), as well as affecting the mammary gland through disturbed supply/drainage via the intercostal artery and vein as well as altered function of the third intercostal nerve.
Just to add some detail I want to point out the possibility of the majority of the brachial plexus being affected by a lesion of the third rib (in the section on the third rib, Clark briefly mentions that the centers for the arm go as low as the fifth dorsal/thoracic segment and that a lesion of the third rib will interrupt the nervous flow between the arm and the central nervous system). As the brachial plexus exits the neck it follows the subclavian artery and vein then the axillary artery and vein as well as travelling between pectoralis minor and serratus anterior. So, the causative factor of a third rib lesion may come from the brachial plexus or more locally from the thoracic segments. Regardless of the causative or reflexive factors, a lesion of the third rib suggests that the arm, rib, and neck offer possible treatment options. If one does not work then the others are possible ways to deal with the third rib lesion as well as any combination of the three. Long lever approaches anyone?