CONNECTED WITH THE SHOULDER- JOINT. 123 



I know, verifiable in the muscle, though it is in the tendon, of the 

 adult. But it corresponds very closely with a description of the 

 posterior layers of the great pectoral in the common rat (Mus 

 clecumanus) ; and it is difficult to see how it is possible to deny the 

 homology of the upper fascicles of the posterior layer of the great 

 pectoral in this animal with the pectoralis minor of man. The 

 nerve-trunk which thus supplies in the bird what I hold to be the 

 homologue of the two pectorales of anthropotomy does, it is true, 

 give off a nerve to one other muscle, viz. the coraco-brachialis 

 inferior, or pectoralis tertius, a muscle which arises from the 

 posterior three-fifths of the outer and inferior edge of the coracoid, 

 and is inserted 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 supplies the coraco-brachialis, and is given 

 off from the same outer cord of the plexus, whence the external 

 anterior thoracic arises. 



By studying the four figures in Plate 3, and the descriptions 

 given of them, a better comprehension of the bearings of my 

 position, that the pectoralis secundus of the bird is homologically 

 identical with the epicoraco-humeral of the reptile, will be gained 

 than could be conveyed by any disquisition, however lengthy, 

 if unaccompanied by illustrations. The name ' epicoraco-humeral' 

 has been given by Mr. Mivart ('Trans. Linn. Soc' vol. xxv. 

 p. 383; 'Proc. Zool. Soc' June 37, 1867, p. 778) to a muscle 

 found both in the Echidna hystrioc and in the Iguana tuberculata, 

 which arises from the precoracoid and epicoracoid in the reptile, 

 and from the epicoracoid in the reptile-like mammal, and is ' in- 

 serted into the summit of the radial tuberosity between the inser- 

 tions of the pectoralis major and deltoid.' It was by recognising 

 in the shoulder-muscles of the crocodile (fig. 3, eh) the homologue 

 of this saurian muscle, and by comparing it, when thus recognised, 

 with the highly simplified pectoralis secundus (fig. 4, eh) of the emu 

 (Dromaius novae hollandiae), that I came to hold the view which 

 I 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 secundus of the Gallinae, pre- 

 pares us somewhat for finding in the emu a pectoralis secundus 





