24 THE VOYAGE OF H.M.S. CHALLENGER. 
each other very much in the details of their muscular structure, though in the fore- 
limb the structure of the biceps and the termination of the tensor patagi tendons, and 
in the hind-limb the presence or absence of the ambiens and the accessories to the 
femoro-caudal and semi-tendinosus, present characters available for taxonomic purposes. 
Anterior Extremity. 
Pectoralis primus.—This muscle is always largely developed in the Tubinares, as 
might have been expected from their great powers of flight. It is peculiar in that it 
is always easily divisible into two quite separate layers superimposed on each other, 
besides which it gives off thin fan-like cutaneous branches. A similar disposition of 
the pectoralis primus in two distinct layers is very characteristic of many of the 
Ciconiiform birds of Garrod, occurring in all the Storks and Cathartide, and in Phaéthon, 
Fregata, Plotus, Sula, and Pelecanus amongst the Steganopodes, A tendency to a 
similar condition, though the two layers are only separable with difficulty, may be seen 
in the Ardeidze, Falconide, and Scopus. 
The superficial layer of the pectoralis primus arises (vide Pl. III. figs. 1 and 4, 
p. 1a) from the posterior and lateral margins of the body of the sternum, from the 
margin of the sternal carina, and from the inferior border and external surface of the 
clavicles. In the latter position it is divisible into two layers, one arising from the 
extreme margin, the other and deeper from the surface, of those bones. The common 
insertion into the large humeral crest is very tendinous behind, more fleshy anteriorly, 
these two parts being somewhat divided by the thick tendon of the deep layer oe the 
muscle (vide Pl. IIT. fig. 1, p. 1a). 
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 furcula, 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 (PI. III. figs. 1 and 2, p. 1b). 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 
