55 © Springer Science+Business Media New York 2016 T.L. Kivell et al. (eds.), The Evolution of the Primate Hand, Developments in Primatology: Progress and Prospects, DOI 10.1007/978-1-4939-3646-5_4 Chapter 4 Morphological Diversity in the Digital Rays of Primate Hands Biren A. Patel and Stephanie A. Maiolino 1 Introduction The primate forelimb consists of three segments distal to the pectoral girdle. The most proximal is the brachium (the arm), which contains the single humerus. The middle segment is the antebrachium (the forearm), which contains two bones, the radius and ulna. The distal segment is the manus (hand), which contains several bones within the carpus (wrist), the metacarpus (palm), and the digits (fingers). Collectively, the metacarpus and digits form the digital rays of the hand. The hand, including the carpus, can account for 20–42 % of total forelimb length across primates (Table 4.1). The rays are numbered 1–5 from radial to ulnar. The first ray is the thumb (pollex), and this is followed by the index (demonstratorius or pointer), middle (medius), ring (annularis), and little (minimus) fingers according to the conventional termi- nology used for the human hand. Historically, each digit has been referred to by vari- ous names, but typical among nonhuman primates the thumb is referred to as the pollex and the remaining rays as ulnar rays 2 through 5. The interested reader is encouraged to see Napier (1980) for a more colorful and detailed history of hand ray nomenclature. The focus of this chapter is on the digital rays of the hand. B.A. Patel (*) Department of Cell and Neurobiology, Keck School of Medicine, University of Southern California, 1333 San Pablo Street, BMT 404, Los Angeles, CA 90033, USA e-mail: birenpat@usc.edu S.A. Maiolino Interdepartmental Doctoral Program in Anthropological Sciences, Stony Brook University, Stony Brook, NY, USA Department of Pathology and Anatomical Sciences, University of Missouri School of Medicine, Columbia, MO 65211, USA