THE EXTERNAL STRUCTURE OF BIRDS. 15! 
swim, have an elongated powerful tail which is their main aid 
in swimming; but no bird whatever swims by its tail. Very 
many climbing animals are assisted by a tail which is ‘* prehen- 
sile” or can grasp. Some birds (such as Woodpeckers) are 
aided in climbing by their very stiff tail-feathers. But no bird 
has a tail which can grasp. No existing bird has a long tail in 
the sense that a Cat has one. That part of a bird which 
answers to the tail of a beast is a short fleshy, more or less 
heart-shaped structure, which in the chicken is often called 
the “‘parson’s nose.” Into it the long true tail-feathers are 
implanted, and it also commonly bears on its upper surface, 
at its root, a peculiar body known as the ot/-gland, sometimes 
called the wropygial gland or the eleodochon. The structure of 
the bony basis of this true tail must, of course, be reserved for 
description along with that of the rest of the endoskeleton. Here 
we are alone occupied with its exterior and its epidermal appen- 
dages. In the first place the oil-gland is composed of numerous 
contorted tubes, which gather themselves together and unite more 
and more till they open by one or several pores on the surface 
generally, on a little papilla. These tubes secrete within them 
a greasy fluid, which exudes and can be pressed out from the 
pore or pores. This gland is specially developed in aquatic 
birds, which carefully anoint their feathers with its secretion, 
the presence of which causes water so proverbially to ‘“ run off 
a duck’s back.” The gland is often surrounded with a circlet of 
feathers, the presence or absence of which serves as a distinctive 
character of various species, and is by some anatomists con- 
sidered important enough to define the great orders. 
The true feather-tail is formed by those generally well- 
developed feathers which are inserted into the fleshy tail. These 
feathers are called vectrices or steerers, and are, as a rule, 
thoroughly firm and pennaceous, though generally the web of 
the outer side of each feather is narrower than the other. The 
rectrices are even in number, and there are generally twelve of 
them. This number may, however, be diminished to eight or 
raised to twenty or four-and-twenty, while the Penguins may 
have two-and-thirty or even more. When the rectrices are 
expanded it will be found that the central pair are inserted 
highest up (most dorsally), one being higher in origin than the 
other. The insertion will be found to follow on alternately— 
the next to the median pair on one side being inserted above 
the next to the median pair of the other side, and soon. These 
