Page 111. 
208 ON CERTAIN MUSCLES OF BIRDS. 
the accessory femoro-caudal is large, being perforated by the sciatic 
artery and nerve ; and the ambiens is strong. 
In Struthio camelus the ambiens is also well developed. 
Part II.* (Plate V.) 
The facts contained in the former part of this communication 
(“‘ Proceedings of the Zoological Society,” 1873, p. 626) being in an 
expanded form, it is not easy to appreciate their full significance at a 
glance, nor without considerable difficulty. To obviate this incon- 
venience I have constructed the following table, which is so arranged 
that by a-very simple method, it is possible to tell without further 
reference which of the five muscles—the ambiens, the femoro-caudal, 
the accessory femoro-caudal, the semitendinosus, and the accessory 
semitendinosus—are present or absent. To obtain this result the 
names of the muscles themselves have been omitted, and single letters 
of the alphabet used in their stead. 
The femoro-caudal is represented by.............. A 
The accessory femoro-candal.................... B 
The semitendinasns 4:9... f. + on c-<s om sgh gree nis x 
The accessory semitendinosus................... § 
When these four muscles are present in a bird the formula 
AB.XY expresses the fact; when any one is absent, that such is the 
case is indicated by the omission of the letter representing it. Thus 
the formula A. XY indicates that the accessory femoro-caudal 
muscle only is absent; AB. X that the accessory semitendinosus is 
missing; A. X that the femoro-caudal and semitendinosus only are 
to be found; and A that the femoro-caudal alone is present. These 
formule may be termed myological formule. No bird is known in 
which all these four muscles are deficient. 
In the accompanying diagram (Plate [5] XVII.) all those birds 
which have the same myological formula are included together in one 
circle; and the circles are so placed in relation to one another that, if 
they were drawn on the surface of a sphere, there would be only asingle 
operation of addition or subtraction necessary to move from any one 
to any of its immediate neighbours. 
Of the four letters A, B, X, and Y the following are the sixteen 
possible combinations. 
* Part II. “ Proceedings of the Zoological Society,” 1874, pp. 111-28, pl. XVII. 
Read, Feb. 3, 1874. 
