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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.

Struthers' ligament
From: Struthers, J. On some points in the abnormal anatomy of the arm.
Br. Foreign Medico-Chir. Rev.13:523-533, 1854.
Struthers' ligament
From Khoo, et al., 1996.

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.

Human humerus X ray of humerus Kangaroo Humerus

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.

Darwin wrote:

"In some of the lower Quadrumana, in the Lemuridae and Carnivora, as well as in many marsupials, there is a passage near the lower end of the humerus, called the supra-condyloid foramen, through which the great nerve of the fore limb and often the great artery pass. Now in the humerus of man, there is generally a trace of this passage, which is sometimes fairly well developed, being formed by a depending hook-like process of bone, completed by a band of ligament. Dr. Struthers,* who has closely attended to the subject, has now shewn that this peculiarity is sometimes inherited, as it has occurred in a father, and in no less than four out of his seven children. When present, the great nerve invariably passes through it; and this clearly indicates that it is the homologue and rudiment of the supra-condyloid foramen of the lower animals. Prof. Turner estimates, as he informs me, that it occurs in about one per cent of recent skeletons. But if the occasional development of this structure in man is, as seems probable, due to reversion, it is a return to a very ancient state of things, because in the higher Quadrumana it is absent.

* With respect to inheritance, see Dr. Struthers in the Lancet, Feb. 15, 1873, and another important paper, ibid., Jan. 24, 1863, p. 83. Dr. Knox, as I am informed, was the first anatomist who drew attention to this peculiar structure in man; see his Great Artists and Anatomists, p. 63. See also an important memoir on this process by Dr. Gruber, in the Bulletin de l'Acad. Imp. de St. Petersbourg, tom. xii., 1867, p. 448."