ON CERTAIN MUSCLES OF BIRDS. 209 
A-BOUSY. AB B.XY XY 
AB.X A.X B.X x 
AB.Y Ay, BY. Y 
A.X Y¥ A B 0 
Those printed in ttalics are those which, if considered as myo- 
logical formule on the nomenclature above given, have been found 
amongst birds; in other words, there are eight different types of Page 112. 
muscular arrangement in the thighs of birds, as far as the four 
muscles now under consideration are concerned. 
Each of the circles in the diagram is divided into two halves bya 
vertical line, so that the names of all those birds which are found to 
possess the ambiens muscle may be distinguished from those in which 
it is absent.. The birds in which the ambiens is present are placed in 
the left-hand semicircles; those in which it is absent are to be 
found in the right-hand semicircles. 
A few examples may illustrate the method of employing this table. 
Taking the Musophagide, for instance, they being found in the circle 
AB. XY evidently possess all the four muscles—the femoro-caudal, 
the accessory femoro-caudal, the semitendinosus, and the accessory 
semitendinosus; and being on the left side, have also the ambiens. 
The Strigide being on the right side of the circle with the formula 
A, possess only the femoro-caudal, the ambiens being also absent, 
and so on. Those birds whose names appear partly in both semi- 
circles may or may not possess the ambiens muscle in their different 
genera. 
Before going further it will be necessary to show what degree of 
constancy is, as a rule, to be expected in the myology of birds. 
Respecting this point my experience is that individuals of a species 
agree precisely in their muscular arrangement. Many specimens of 
a considerable number of species have passed through my hands, and 
I have frequently dissected them one after another in order to detect, 
if possible, any individual variations; such, however, have not been 
forthcoming. It is true that in a single specimen of (dicnemus 
grallarius the ambiens, instead of crossing the knee, ended on the 
inner side of the ligamentum patella; however, the muscle, though 
imperfectly developed, was undonbtedly there. The only other in- 
stance of an unexpected and abnormal individual structure was the 
presence in a specimen of Pomatorhinus temporalis, on one side only, 
of an accessory femoro-caudal, which I have not once seen in any 
other of at least 100 Passerine birds that I have examined. 
From these observations it is evident that individuals of a species 
all agree in the arrangement of the muscles of the thigh at least—so 
much so that any peculiarity observed in a specimen dissected for the 
first time may be taken to be characteristic of the species itself. 
P 
