MACALISTER— ON PRONATOR MUSCLES. 219 
in the Iguana tuberculata, in which it forms the pronator accessorius 
so accurately described by Mr. Mivart. In its insertion, this muscle 
usually seeks the radius above the pronator teres, but in all the fore- 
mentioned cases its radial attachment is extended to touch or even 
to overlap the pronator quadratus; the cause of separation in these 
instances seems to be the necessity for an ulnar origin for the flexor of 
the digits. A further divergence from the type occurs in Gongylus 
ocellatus, in which this pronator accessorius 1s connected to the pro- 
nator teres, and separated from it by the median nerve—a state which 
directly conducts us to the arrangement of the two-headed pronator 
teres of man (for further observations on which, see vol.1., N.S. of 
the ‘“‘ Journal of Anatomy,” p. 8, e¢ seg.). This muscle is entirely 
obsolete in Solipeda, Proboscidea, Ruminantia, Cheiroptera, Cetacea, 
the hyrax, hare, rabbit, armadillo, &c. 
There are some interesting varieties of arrangement in the human 
subject of these muscles. Among the most striking of these is the form — 
of pronator quadratus, in which the lower fibres arose from the lower 
seventh of the ulna, and formed a round belly, whose tendon passed 
across the lower end of the radius to be lost in the aponeurotic struc- 
tures over the scaphoid, trapezium and trapezoid bones (‘ Proc. Royal 
Irish Academy,” 1867). In a fine Bengal tiger, dissected in February, 
1869, the lower fibres passed over the inferior extremity of the radius, to 
be inserted into the tendon of the extensor ossis metacarpi pollicis. 
In some animals, as the chameleon, the segmentation of the super- 
ficial pronator teres is almost complete, and its fibres form two nearly 
separate bellies. This throws light upon the fission in the avian pro- 
nator, as the coexistence of a split superficial pronator over the median 
nerve with the pronator accessorius shows that the deep avian muscle 
is not necessarily of a ditferent type from its superficial neighbour, 
both lying over the median nerve. The homotypes of these muscles 
are by no means clearly defined. Meckel and Huxley regard the pop- 
liteus as representing the pronator teres; and it has likewise been 
surmised that the tibial head of the soleus represents the quadrate 
pronator. The former of these homologies is by no means unobjec- 
tionable; for, leaving out of account the question of direction, or of 
the interpretation of the upper parts of the tibia and fibula, there 
are the following objections to the hypothesis :—the origin of popliteus 
is from the outer condyle, that of pronator teres from the inner; the 
former arises by a long tendon; the latter, as far as I am aware, never 
does. The former lies deeply seated in contact with the joint; the 
latter les the most superficial of the ento-condyloid group of muscles; 
the former lies beneath the popliteal nerve, the latter lies superficial 
to the median nerve; the former crosses above the point of perforation 
of the anterior tibial artery, the latter crosses on a plain below the 
origin of the posterior interosseous artery. These considerations seem 
to abnegate the hypothesis of Meckel, and lead us to look elsewhere 
for a muscle fulfilling the required conditions. As pronation is the 
VOL. V. 26 
