Chick vincula: elastic structures with a check-rein mechanism.

From their mode of attachment and their elastic composition, it is clear that the vincula of the chick serve other functions besides that of carrying blood vessels to the digital flexor tendons within their synovial sheaths. Evidence is presented in support of the argument that elastic fibres bear the brunt of rapidly applied tensile forces and that the interweaving collagen fibres only become taut when the vincula are stretched to the limit and about to tear. Our hypothesis is that the collagen serves as a check-rein mechanism in an otherwise elastic structure.