The Coraco-Acromial Ligament and Projection Index in Man and Other Anthropoid Primates
Total Page：16
File Type：pdf, Size：1020Kb
J. Anat. (1977), 124, 3, pp. 627-632 627 With 2 figures Printed in Great Britain The coraco-acromial ligament and projection index in man and other anthropoid primates RUSSELL L. CIOCHON AND ROBERT S. CORRUCCINI Department ofAnthropology, University of California, Berkeley, CA 94720 U.S.A. and Division ofPhysical Anthropology, Smithsonian Institution, Washington, D.C. 20560 U.S.A. (Accepted 11 October 1976) INTRODUCTION Studies on the functional anatomy of the primate shoulder (Keith, 1894, 1923; Miller, 1932; Ashton & Oxnard, 1963; Washburn, 1968; Morbeck, 1972; Corruccini & Ciochon, 1976) have shown a number of specialized morphological features in Hominoidea which differ markedly from the presumed ancestral pattern typified by the condition in cercopithecoid monkeys. These differences have been correlated with a radical shift in the locomotor/feeding adaptations of hominoid primates with which the term 'brachiation' has been associated by many authors in the past. One particular, heretofore relatively neglected, anatomical feature of the shoulder of hominoids is the presence of a strong triangular ligament extending from the tip of the costal surface of the acromion (lateral to the clavicular facet) to the whole postero-superior border of the coracoid process. The coraco-acromial ligament acts with the acromion and coracoid process to limit superior movement of, and to serve as a protective cuff for, the head of the humerus; it also separates the deltoid and supraspinatus muscles which abduct the humerus. A homologue of this ligament has never been detected in lower anthropoid primates (i.e. cercopithecoid and ceboid monkeys), raising the possibility that the ligament is unique to the Hominoidea (including, of course, man). The objects of the present study are to present a few new observations on the comparative presence or absence of the coraco-acromial ligament, and to evaluate the functional significance and variance of an associated mensural trait, the coraco- acromical projection index. MATERIALS AND METHODS Visual observation of a large series of prepared hominoid, cercopithecoid and ceboid scapulae revealed that the coracoid and acromion processes of the Homi- noidea are relatively the largest and project far laterally from the plane of the glenoid cavity. The enlarged acromion in hominoids is correlated with a much thickened deltoid muscle which originates from it and is inserted far distally on the humerus. Pectoralis minor is inserted on the enlarged laterally projecting coracoid process rather than on the humerus, as is the case in the lower anthropoid primates. Together these processes provide both an advantageous biomechanical arrangement for deltoid and trapezius and an anchorage for the strong coraco-acromial ligament. 628 R. L. CIOCHON AND R. S. CORRUCCINI In particular, deltoid is afforded a powerful angle of pull during abduction without the possibility of its dislocating the humerus in a vertical direction. Along with the modification of these scapular processes in the Hominoidea, changes in the glenoid cavity have also occurred: thus hominoids have a relatively wide and flat glenoid cavity, with a non-projecting supraglenoid tubercle. In hominoids also the humeral head is well-rounded and expanded, relative to the slender shaft, in the antero-posterior dimension. These modifications, when coupled with the above-mentioned muscular rearrangements, allow a much greater range of 0,0 / ~~~~~~~~~~~4, Fig. 1. Pictorial representation of the coraco-acromial projection viewed from the superior aspect of the scapula. The scapula of Homo sapiens is utilized here to depict the hominoid condition. circumduction, with the ability to move and reach in all directions greatly enhanced. Conversely, the Cercopithecoidea and most of the Ceboidea are characterized by non-projecting coracoid and acromion processes and absence of a coraco-acromial ligament; a narrow glenoid cavity which is concave, with a projecting supraglenoid tubercle; and a humeral head which is much elongated in the medio-lateral dimen- Coraco-acromial projection index 629 sion.* This morphology reflects a much more fixed and stable gleno-humeral joint than in hominoids, with arm movement restricted to a partial arc of flexion and extension. In order to quantify the observed qualitative differences in the shoulders of hominoids, cercopithecoids and ceboids we have devised the following interval scale measurement, termed the coraco-acromial projection, which is defined as the minimum perpendicular distance from a line connecting the most lateral points on the coracoid process and the acromion to the most lateral projection of the supra- glenoid tubercle. This is recorded as a negative value in cases where the supraglenoid tubercle projects further laterally than either the coracoid or the acromion. Figure 1 illustrates this measurement. In order to cancel out some of the effects of body size on the measurement's gross magnitude we found it necessary to employ a ratio of the value. The height of the glenoid cavity is used as a divisor in this ratio and is defined as the distance between the most superior extension of the glenoid articular surface at the supraglenoid tubercle and the most inferior extent of the articular surface immediately superior to the infraglenoid tubercle taken at the glenoid cavity midline. These measurements were made on 433 scapulae representing 20 species of Anthropoidea. The specimens were studied at 15 museums in Great Britain, Europe and North America. There were five species of Hominoidea, eight species of Cercopithecoidea and seven species of Ceboidea (see Table 1). We selected these species because they exhibit different locomotor-behavioural repertoires and because they represent the major taxonomic groupings within the Anthropoidea. Approxi- mately equal numbers of each sex were obtained for each species in order to balance any possible sample skewing effects due to sexual dimorphism. Approximately 90 % of the measured specimens were caught in the wild. The remaining 10 % were from zoological gardens and laboratories because of the lack of wild-caught representa- tives of certain species. All specimens in the study were examined for bone disease, healed fractures, gunshot wounds, arthritic joint surfaces and other abnormalities. Only adult specimens (as judged by fully fused epiphyses) lacking obvious pathology were selected. RESULTS Table 1 summarizes the metric results of the coraco-acromial projection, glenoid height and coraco-acromial projection index measurements for all the species of Anthropoidea in our comparative sample. These basic statistical results are fully compatible with additional observations on about 600 prepared scapulae and some wet dissections of new taxa. Figure 2 displays the metric results presented in Table 1. A complete separation between hominoids and cercopithecoids is evident. There is almost as much variance between these two groups as there is within each of them. The ceboid monkeys, however, appear to be much more generalized. The outer limit of the ranges of their coraco-acromial indices overlap with both the hominoids and the cercopithecoids. Except for the howler monkey, Alouatta, ceboids uniformly display much less intragroup variance than either the Hominoidea or the Cercopithecoidea. Alouatta's * All topographical references used in this study refer to the standard human anatomical position. All non-human specimens were placed in this position before measurements were made. In addition, all descriptive anatomical nomenclature used here conforms with the terminology adopted in Nomina Anatomica 3rd edition (1966). 630 R. L. CIOCHON AND R. S. CORRUCCINI Table 1. Coraco-acromial projection and index in selected Anthropoidea Coraco-acromial projection Glenoid height Index Taxon (N) Mean S.D. Mean S.D. Mean S.D. S.E. Hominoidea Homo sapiens (26) 19-42 2-72 35-43 3-45 0 55 0-07 0-027 Pan troglodytes (23) 20-88 1-90 32-91 2-62 0-64 0 09 0-038 Gorilla gorilla (22) 31-32 6-73 50-19 6-84 0-62 0-08 0 034 Pongo pygmaeus (24) 17-22 2-62 37-11 5 09 0 47 0-08 0 033 Hylobates lar (22) 12-60 0 97 14-16 0-98 0-89 0 09 0-038 Cercopithecoidea Colobus guereza (22) 3-47 0-66 15-93 1-17 0-22 0-04 0-017 Nasalis larvatus (21) 5-14 1-07 20-24 2-78 0-25 0-05 0-022 Presbytis rubicunda (22) 3-11 0 35 13-55 0-66 0-23 0-02 0 009 Cercopithecus mitis (22) 2-43 0-75 14-95 1 54 0-16 0 05 0-021 Erythrocebus patas (14) 1-71 0-54 17-04 2-06 0-10 0-04 0-021 Macaca fascicularis (21) 1-87 0 59 12-84 1-45 0-14 0-04 0-017 Papio cynocephalus (22) -0-18 1-23 24-45 3-54 -0-01 0-05 0-021 Theropithecus gelada (24) -1-30 0-62 21-25 2-26 -0-06 0 03 0-012 Ceboidea Ateles paniscus (23) 9-62 1 19 15-96 1-26 060 006 0025 Lagothrix lagothrica (21) 6-51 0-81 16-22 1 22 0 40 0 05 0-022 Alouatta villosa (24) 4-42 0-94 16-06 1-39 0-28 0 05 0-020 Cacajao calvus (15) 6-67 1 31 13 34 1-34 0 50 0-07 0-036 Pithecia pithecia (18) 4-66 0-76 10 09 0.99 0-46 0 07 0 033 Cebus apella (24) 4-78 1-05 11-72 1-12 0 41 0 07 0-029 Saimiri sciureus (23) 2-88 0-41 7-47 0-42 0-39 0-06 0-025 Hor7o sapiens a Pan troglodytes m GorY/Ja gorilla - Pongo pygmaeus Hylobates lar Colobtus guiereza a Nasalis larvatus a Presbytis rujbicujnda a Cercopithecus tnitis Erythrocebus patas a Macaca fascictularis a Papio cynocephalus n TheropithecLus gelada Ate/es paniscuis ± Lagothrix lagothrica w Alonatta vil/osa Cacajao calvuJs Pithecia pithecia Cebus ape/la Saimiri sciureus I I I I I 1 --0 40 -020 0 0 20 0 40 0 60 0 80 1 00 1 20 Coraco-acromial projection indcx Fig.