THE VOYAGE OF THE * CHALLENGER.' 389 



these two parts being somewhat divided by the thick tendon of the deep 

 layer of the muscle (vide PL XVI. fig. I, p. la). 



The deep layer of the pectoralis primus arises chiefly from the body 

 and keel of the sternum outside the origin of the pectoralis secundus 

 from which it is separated by a strong fascia from the tip of the 

 f urcula, and from the fascia over the second pectoral, especially anteriorly, 

 where a large air-space separates these two muscles in the interval 

 between the furcula and coracoid (PL XVI. figs. 1 and 2, p. 16). Its 

 tendon is thin anteriorly, strong and cylindrical posteriorly, and is 

 inserted, as already described, between the two parts of the tendon of 

 the superficial layer which arches over it. 



The muscle is perforated a little anteriorly to its posterior border, and in 

 front of the strong tendinous band dividing it, by a group of vessels and 

 nerves destined for the supply of the muscles and skin incumbent on it. 

 The most posterior of its fibres do not apparently join the main tendon 

 of insertion, but are lost in the loose fibrous tissue occupying the axillary 

 region. 



There is a large cutaneous branch given off by the superficial layer 

 close to its insertion, which runs back over the humerus, and is distributed 

 as a fan-shaped expansion to the outer branch of the pectoral tract. 

 Another cutaneous branch comes off from the anterior end of the muscle Zool. Chall. 

 on the breast close to the sympliysis furculce, and goes to the skin of the Exp. vol. iv. 

 lower and anterior aspect of the neck. 



Pectoralis secundus. This muscle is also well developed, but though 

 broad is usually short, extending for not more than one third, one half, 

 or sometimes two thirds the length of the sternum. In Pelecano'ides, 

 however, it is much longer, extending to nearly the end of that bone, and 

 in Procellaria, Garrodia, Fregetta, and Pelagodroma its extent is nearly as 

 large, in which cases it extends beyond the posterior margin of the deep 

 layer of the first pectoral. 



It arises from the antero-superior part of the carina sterni, and from 

 the body of the bone external to that, from the greater part of the 

 coraco-furcular membrane below the pectoralis tertius, from the symphysis 

 furculce, and from a greater or less extent of the antero-inferior border 

 of the coracoid bone. Its insertion is by the usual tendon on the 

 superior aspect of the humerus, behind the much smaller tendon of the 

 third pectoral. 



In the Albatrosses the pectoralis secundus is unusually short, and 

 broken up into four quite separate parts, which unite before passing the 

 shoulder-pulley. This arrangement is clearly shown in fig. 2 of Plate 

 XVI., representing the muscle in Diomedea brachyura. In the other 

 Petrels, the muscle is much more homogeneous, and only separable by 

 dissection into its various component parts. 



Pectorali* tertius. This muscle (PL XVI. fig. 2, p. 3) is always well 



