Willem Kramer

“The toe bone connected to the heel bone, the heel bone connected to the foot bone, the foot bone connected to the leg bone, the leg bone connected to the knee bone, the knee bone …”

Like the song “Dry Bones” by James Weldon Johnson implies, the bodies’ bones connect with one another. What James Johnson did not sing about is that not only bones but all our organs connect. Our arteries, bones, deep fasciae, ligaments, muscles, peripheral nerves, and all other organs physically connect to form the human body.

Although physical inter-organ connections have unique features based on which organs connect and where the connection is located in the body, we can divide them into two large groups; the physical fixed connections and the physical not fixed connections.

The fixed connections are, like their name implies, tightly anchored. They do not allow – or are not supposed to allow – any movement between connecting parts. To guarantee a strong link, the fixed connections are mainly established through the organ-unique connective tissues that invest and surround them. For example, the epineurium, perineurium and endoneurium of peripheral nerves are continuous with the meninges (the dura, arachnoid, and pia mater) surrounding the CNS. In the periphery, the nerves are anchored to the muscles through the “same” epineurium, perineurium and endoneurium. Bones and muscles also connect with one another. At a muscular attachment site, the bone’s periosteum is continuous with the epitenon and endotenon of the muscular tendon. For extra strength the tendon’s collagen bundles are anchored deep into the (cortical) bone. From a functional perspective, the fixed union – no freedom or slack between connecting parts whatsoever – is a requirement for health and proper function.

Opposed to the fixed links, the not fixed connections are not tightly anchored. They do allow – or are supposed to allow – at least some movement between connecting parts. The not fixed connections are generally established through interposed loose connective tissue. Arteries, veins and peripheral nerves, for example, loosely connect with muscles and the other organs they cross through a thin layer of loose connective tissue. Interposed loose connective tissue also ensures some movement between neighboring muscles. Of all not fixed connections only the bone-bone links (also known as synovial articulations or joints) are not established through loose connective tissue. Unlike other not fixed bonds these joints are established through interposed synovial fluid. From a functional perspective, the “free” or not fixed relationship – a certain degree of kinetic freedom or slack between connecting parts – is a requirement for health and proper function.