REPORT ON THE MARSUPIALIA. 51 



of the dorsal interossei. Fusion of the constituents of the intermediate and dorsal layers 

 is extremely common, whilst fusion between the plantar and intermediate muscles is a 

 very rare occurrence. Suppression of certain of the muscles may take place in two ways. 

 It may either be complete — not a trace of the lost muscle being left — or partial, in which 

 case the place of the missing muscular belly is taken by a ligamentous structure, having the 

 same connections, and probably a distinct function to play in the mechanism of the foot. 



II. The presence of an opponens muscle is not accounted for in the foregoing disposi- 

 tion of the intrinsic muscles. When present, it may be regarded as being derived from 

 one or two sources. Most commonly it is a development from the flexor brevis, but it 

 may proceed, as in many of the Carnivora, from the plantar layer, and thus be associated 

 with the adductors. 



III. The last point which I am anxious to prove is, that in many animals, the relation 

 of the intrinsic muscles to the metatarsal bones, both as regards their origin and position, 

 corresponds with transitory conditions in the foot of the human embryo. 



Let us take up the first of these generalisations, and examine the facts upon which 

 it is founded. 



Marsupialia. 



Thylaeine (PI. VI. figs, l , 2). 



The pes of this animal closely resembles that of the Dog. The hallux is suppressed, 

 but in connection with the remaining four digits the typical disposition of the intrinsic 

 muscles in three layers is very manifest. 



The plantar layer (p 2 , p 4 , p 5 ) consists of three muscles, which have as their function 

 the adduction of the toes, towards a line drawn through the medius. They are — 



1. Adductor indicis (p~). 2. Adductor annularis (/> 4 ). 



3. Adductor minimi digiti (p 5 ). 



The adductor indicis and the adductor minimi digiti arise in the middle line of the 

 foot by a single pointed origin from the fibrous structures at the base of the metatarsus. 

 Separating and then diverging from each other the former is inserted upon the fibular 

 side of the base of the proximal phalanx of the index, whilst the latter is inserted upon 

 the tibial side of the base of the corresponding phalanx of the minimus. In both cases 

 the insertion is partly into the sesamoid bone, and partly into the extensor tendon on 

 the dorsum of the digit. In some cases the adductor indicis is double (vide fig. 1). 



The adductor annularis springs from the base of the middle metatarsal, under cover 

 of the two preceding muscles, and swelling out into a fusiform belly, it is inserted upon 

 the tibial side of the proximal phalanx of the annular digit in a manner similar to the 

 other adductor muscles. 



