AKT.13 BRACHIAL, FLEXORS IN PRIMATES HOWELL AND STRAUS 19 



of the common coracoid tendon, or else from the coracoid proper 

 posterior to the tendon. Insertion is invariably upon the neck of 

 the humerus either entirely proximal to, and back of, the tendon of 

 the latissimus dorsi, as we found, for example, in the potto {Pero- 

 dicticus), or else overlapping the tendon by only a couple of milli- 

 meters, as we found in the macaque {Plthecus). Occasionally it is 

 extremely slender, as in the spider monkey (Ateles) that we dissected. 



According to Parsons (1898) this slip is present in some genera 

 and absent in others of the marsupials, edentates, cetaceans, ungu- 

 lates, rodents, carnivores, insectivores, and primates, and is always 

 absent in bats. Parsons and other authors give long lists of the 

 genera concerned, but there is no need to repeat these lists here. 

 Among the primates, however. Parsons stated that it is present in 

 more than 30 per cent of chimpanzees but is less frequent in gorillas 

 and orangs, while it is always present in lemurs. Keith (cited by 

 Parsons, 1898) reported it as rarely absent in cynomorphous 

 monkeys. 



A survey of the available literature reveals the fact that this 

 muscle is present regularly in lemurs and Tarsius, is usually present 

 in both catarrhine and platyrrhine monkeys, and occurs only very 

 occasionally in anthropoid apes other than the chimpanzee.^ 



Apparently, then, this structure is present in some genera of all 

 primate families but is absent in others, and so it is not a very im- 

 portant phylogenetic index; but whether its significance is purely 

 functional it is difficult to say. Parsons (1898) suggested that the 

 suppression of this muscle might be attributable to, or correlated 

 with, the setting back of the shoulders. In general, however, it is 

 usually present in the more primitive primates and tends toward 

 complete disappearance in the so-called " higher " forms. The im- 

 portant point is that when it occurs at least most of its fibers are 

 located entirely proximal to, and usually deep to (behind), the 

 latissimus tendon. Therefore it is usually separated from the pars 

 media by the latter tendon. 



The coracobrachialis medius, or " proprius," always originates 

 from the coracoid, and it can always be identified by this fact, 

 together with the circumstance that it inserts upon the shaft of the 

 humerus, although occasionally the relationship of its terminal fibers 

 with those of the medial triceps is extremely close (as in Cehus and 

 Perodicticus) . Otherwise it is quite variable. Origin is usually 



8 According to the data that we have collected from the literature and to which we 

 have added the results of our own dissections, the coracobrachialis profundus (brevis) 

 has been found in 1 out of 10 gorillas (10 per cent), 4 out of 13 chimpanzees (31 per 

 cent), and 1 out of 10 orang-utans (10 per cent). It was not found among 8 gibbons. 

 We ourselves have met this structure, but in various degrees of development, in Pithecus, 

 Papio, Lasiopyga, Ceius (first specimen), Saimtri, Ateles, Aotus (two specimens), Oedi- 

 pomidas, Oalago, Nycticeius, Perodicticus, and Tarsius (two specimens), but not in 

 Pan, HyJotates, Pygathruc, and Alouatta, nor in our second specimen of Cehus. 



