Charles Darwin and Struthers' liagament
In Chapter 1 of The Descent of Man, Darwin deals with organs that, whilst fully functional in other animals, appeared to him to be vestigial and largely functionless in the human body. This he regarded as good evidence that human beings and other kinds of animals share a common ancestry.
One of the organs that he discusses is the so-called Ligament of Struthers (labelled 'b' in the diagram), a fibrous band extending from a large bony projection of the humerus, known as the supracondylar process (labelled 'a'), to the median epicondyle. These structures are present in less than 1% of humans.
In a variety of other mammals, including the marsupials and many carnivores, the median nerve and brachial artery of the arm pass through an opening in the lower part of the humerus, known as the supra-condyloid foramen. This opening is not present in humans and the nerve and artery simply pass over the surface of the humerus. However, in about 1% of humans there is a bony projection, the so-called supracondylar process, on the inside aspect of the lower end of the humerus. Struthers interpreted this, togther with the associated ligament, as a vestigial version of the supra-condyloid foramen. Certainly, the medial nerve and brachial artery do run through the opening that they create.
The supracondyloid process, and its gentic inheritance, were subjects of special interest to Struthers and he wrote four papers on them between 1848 and 1881. These papers came to the attention of Darwin whilst he was writing his The Descent of Man.
The photograph on the left shows a human humerus, prepared by Struthers himself, with a supracondylar process. The middle one is a modern x-ray showing a very large process. That on the right shows the elbow joint of a red kangaroo; the supra-condyloid foramen is clearly visible in the lower part of the humerus. The structures are indicated by red circles.