REPORT ON THE MARSUPIALIA. 137 



by removing the first dorsal interosseus, or indeed by merely dividing the head of this 

 muscle which springs from the first metacarpal bone. It is a very small muscular slip 

 which, as Bischoff expresses it, has been " thrust into the deep," by the largely developed 

 adductor obliquus. It lies in series with the palmar interossei. 



In the tabular view, which is given in the eighth edition of Quain's Anatomy, of 

 Muscular Homologies in the Upper and Lower Limbs, the relations between the muscles 

 of the thumb and great toe are stated with tolerable accuracy with the exception that 

 the position of the two heads of the flexor brevis pollicis is reversed. The following is 

 an extract from that table : — 



Flexor brevis pollicis, inner head, . . Flexor brevis hallucis. 

 Flexor brevis pollicis, outer head, . . Adductor hallucis obliquus. 

 Adductor pollicis, ..... Transveisalis pedis. 



In place of this the table should be constructed thus : — 



Flexor brevis pollicis, .... Flexor brevis hallucis. 



(a) Radial head, ..... (a) Tibial head. 



(6) Interosseus primus volaris, . . (b) Fibular head. 



Deep or ulnar head of flexor } _ Adductor obliquus hallucis. 



brevis pollicis, . . . ) 



brevis pollicis, 

 Adductor pollicis, 



Adductor transversus hallucis 

 (i.e. trans versalis pedis). 



II. The question now arises : If the intrinsic muscles of the foot are laid down in 

 three layers, to which of these does the opponens muscle belong ? 



It is necessary, however, befoi-e we inquire into this point that we should have a clear 

 understanding what the muscles are to which the term " opponens " should be applied. 

 We need not look to function for a true definition, for many of the opponens muscles 

 have little or no opposing action. It is clearly the insertion which must be taken as the 

 distinguishing feature, and we may define the term as being one which may be properly 

 applied to any intrinsic muscle, which is inserted into the shaft of a metatarsal bone. 



An opponens hallucis is a very rare occurrence. In his memoir upon the Hyldbabes 

 leuciscus Bischoff remarks that among the Apes he has " only found it as a peculiar 

 speciality in the Orang and Macacus." In the report of Lecture xvii. upon the Structure 

 and Classification of Mammalia, delivered by Professor Huxley before the Royal College 

 of Surgeons in 1864, and published in the Medical Times and Gazette, we find the 

 following passage (vol. i. p. 596): — "There was present in the specimen (Orang), dissected 

 by Professor Huxley, an opponens hallucis, inserted into the middle third of the meta- 



(ZOOL. CUALL. EXI\ — I'ART XVI. — 1SS2.) Q 18 



