ON CERTAIN MUSCLES OF BIRDS. 191 
in front of the patella to the outer side, where it joins the fibres of 
origin of the flexor perforatus digitorum. 
Femoro-caudal.—This long ribbon muscle is covered superficially 
by the tensor fascis and biceps above, as well as by the semitendi- 
nosus lower down. The sciatic artery and nerve cross it superficially 
at right angles close to its insertion as they course from the sciatic 
foramen, parallel to the femur, to the popliteal region. The femoral 
vein separates this muscle from the adductor muscles at their inser- 
tions, except in Dacelo, where it crosses the femoro-caudal superficially.* 
It arises from the (anterior) transverse processes of the two last 
coccygeal vertebrx, and is inserted into the linea aspera of the femur, 
at about one-third its length from the trochanter. 
An accessory head, arising from the upper thrée-fourths of the 
postacetabular ridge, and from the ridge which forms the lower 
margin of the origin of the obturator externus, joins the tendon of 
insertion of this muscle, and is also partly inserted into the linea 
aspera, between it and the head of the femur. It is thin, muscular, 
and broad, covering the obturator externus superficially, and is par- 
tially intersected by a fibrous sheet where it crosses its anterior 
border. The sciatic artery and nerve cross it superficially ; and the 
nerve to the semimembranosus is deep of it, whilst that to the semi- 
. tendinosus is superficial in some cases; the biceps completely 
covers it. 
Of the above-described muscles, five of them (the ambiens, the 
femoro-caudal, the accessory femoro-caudal, the semitendinosus, and Page 630. 
the accessory semitendinosus) vary; any one or more than one may 
be absent in different birds; and in my dissections my object has 
always been to record the conditions existing in the specimen under 
examination. The constancy of the peculiarities in the different indi- 
viduals of each species, in the species of each genus, and very ~ 
generally in the genera of each family, makes it evident to any one 
working at the subject, that much respecting the affinities of the 
different families of birds is to be learnt from the study of their 
myology, in connexion with the peculiarities of their other soft 
parts; and that these features will, in the long run, lead to a more 
correct classification than one based on the skeleton alone, becomes : 
almost equally certain. 
The variations in the five above-mentioned muscles form the 
subject of this communication, and the subjoined list contains the 
results arrived at by myself. A few of the facts now recorded will be 
found mentioned in the works of Meckel, Sundevall, and others. Re- 
* In Centropus phasianus the main artery of the leg is also the femoral, and not 
the sciatic, as in other birds ; it therefore runs with the femoral vein in that bird. 
