ON THE ANATOMY OF FISHES. 
87 
where also their union to form the main trunks takes place. The ganglia on the 
dorsal roots of the nerves are likewise contained within the sac. 
The Air-Bladder . — On examining the external surface of the body beiiind the 
pectoral girdle and dorsad to the origin of the pectoral fin a well-defined area of 
skin, generally slightly wrinkled or flaccid in spirit-preserved sj^ecimens, sometimes 
extremely tense, but always differing in appearance from the rest of the smooth scale- 
less skin, may be readily recognised on each side (fig, 15, l.c.a.). These areas we 
propose to call the lateral cutaneous areas. Each area is bounded by, and more or 
less firmly attached to, the stem of the post-temporal and the upper portion of the 
clavicle in front ; the posterior process of the clavicle and the free dorsal edge of the 
ventro-lateral muscles below ; and the distal margins of the expanded transverse 
processes, with the first rib, above and behind. The lateral line canal crosses the 
upper margin of each area a little below the dorsal attachment of the latter to the 
modified transverse processes. On reflecting either cutaneous area the great dorso- 
lateral and ventro-lateral muscles of the body wall are seen to diverge at its posterior 
boundary, the former passing upwards and forwards over the expanded transverse 
processes to their insertion anteriorly into the posterior face of the skull, the latter 
passing downwards and forwards to their insertion into the ventral moiety of the 
clavicle (fig. 16). Between these divergent muscles the lateral wall of the anterior 
third of the air-bladder bulges outwards, and, normally, is very closely applied to the 
inner surface of the lateral cutaneous area of its side. The cutaneous areas are some- 
what thinner than the rest of the skin, and their flaccidity, due perhaps to their 
greater thinness, combined with their intimate relations to the thin-walled air- 
bladder, renders it, as a rule, an easy matter to localize these areas without having 
recourse to dissection. In not a few Siluroids, as we shall subsequently have occasion 
to point out, in which the body is greatly compressed, and the cutaneous areas and 
the adjacent lateral walls of the air-bladder exceptionally thin, this part of the body 
appears partially translucent when the fish is held up to the light. 
When the ventral wall of the body with the stomach, liver, and intestines have been 
removed the air-bladder comes into view as a flattish conical organ, broad in front, 
but somewhat narrower and more rounded behind (fig. 17, a.h.). The dorsal surface 
of the anterior third of the bladder is closely moulded to the shape of the ventral 
surfaces of the modified transverse processes of the fourth and fifth vertebrae, and to 
the keeled ventral surfaces of the complex and fifth vertebral centra, but for tlie 
posterior two-thirds of its length the bladder is much less closely related to the 
succeeding vertebral centra, transverse processes, and ribs, and lies comparatively free 
in the abdominal cavity. In front tlie lateral portions of the anterior wall of the 
air-bladder are firmly buttressed by the concave posterior faces of the bony structures 
formed by the post-temporal plates in conjunction with the crescentic extremities of 
the modified transverse processes of the fourth vertebra (figs. 3, 17, 18, tp.hi.). 
Tlie lateral walls of the anterior third of the bladder are in close relation with the 
