MUSCLES CONNECTED WITH THE SHOULDKR-JOINT. GVJ 



or pectoralis terlius, a muscle which arises from the posterior three-fifth* of the 

 outer and inferior edge of the coracoid, and is insert. d into the inner tuberosity 

 just inside the most mesially placed portion of the crateriform rim which it throws up 

 round the mouth of the pneumatic inlet. But this nerve-trunk seems to me to be the 

 homologue of the musculocutaneous nerve of anthropotomy, which 

 brachialis, and is given off from the same outer OOttd of the plexus, 

 anterior thoracic arises. 



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By studying the four figures appended to this paper, and the descriptions given of 

 them, a better comprehension of the bearings of my position, that the jwctortUii §ecundus 

 of the bird is homologically identical with the ep i eoraco-hnmeral of lite reptile, will be 

 gained than could be conveyed by any disquisition, however lengthy, if unaccompanied 

 by illustrations. The name " epicoraco-hurneral " has been given by Mr. Mi\art (Trans, 

 Linn. Soc. vol. xxv. p. 383; Proc. Zool. Soc. June 27, 1*07, p. 778) to a muscle found 

 both in the Echidna hystrix and in the Iguana tuherculata i which arises from the pr< o- 

 racoid and epicoracoid in the reptile, and from the epiooracoid in the reptile-like 

 mammal, and is "inserted into the summit of the radial tuberosity between the inser- 



tions of the pectoralis major and deltoid." It was by recognizing in the shoulder- 



muscles of the Crocodile (fig. 3, eh) the homologue of this Saurian muscle, and by 

 comparing it, when thus recognized, with the highly simplified pectoralis sccumlns (tig. 

 4, eh) of the Emu (Dromaius Novce Holland We), that I came to hold the view which 1 

 now put forth. The great reduction, already spoken of, which the sternal origin of the 

 accipitrine levator humeri has undergone, as compared with the vast pectoralis secuudus 

 of the Gallinae, prepares us somewhat for finding in the Emu a pectoralis sccundaa 

 which, while it coincides in insertion, differs little in origin from the epicoraco-humcral 

 of the Crocodile, except in the possession of a small head from the middle point of the 

 sternum. The essential relations held by the muscles lettered eh in each of the two 

 figures 3 & 4, to the surrounding parts, seem to be much the same in the intervals 

 between their origin and insertions. The crocodilian muscle, however, receives a head 

 from the visceral surface of the scapula, in compensation for the absence of the fibres 

 which in the Emu arise from the coraco-clavicular membrane and sternum, but which 

 in the Reptile appear to have been displaced by its great pectoral from its tess- 

 developed sternum. The pectoralis secundus of the Sparrow- Hawk, as already de- 

 scribed, receives in a similar way an accession of fibres from the viscerad surface of the 

 coracoid and sternum. It is possible that the fascicle ei in the Crocodile, if the homo- 

 logue of a dislocated anterior segment of rectus abdominis may foreshadow the sternal 

 prolongation of the avian levator humeri. But though this muscle seems exceedingly 

 ready to build up extraneous elements into its own mass, it is perhaps more easy, with 

 our knowledge of this muscle as it is to be described hereafter in the Prog, to con- 

 ceive the epicoraco-hurneral as encroaching from its own area on to that of the sternum, 

 than to conceive of it as fusing with a "rectus thoracis," even though the subclaim s of 



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the Wombat does actually do this. Another point of difference is presented by the reten 

 tion in the Emu of a rudiment of the coracoidal pulley, which is not indicated in tin 

 Crocodilian shoulder-girdle, either as a nascent or as a retrograding structure. The pec 



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