REPORT ON THE MARSUPIALIA. 131 



favour of looking upon the dorsal muscles, as derived secondarily from the flexors. Fixity 

 of structure is an impossibility. There are few cases, indeed, in which, either by com- 

 parative or developmental research, anatomists are able to penetrate so far into the 

 obscurity which surrounds the history of the changes which structure undergoes as to 

 found an absolute fundamental type. It is very cmestionable whether such is possible 

 in any case. The three chief movements of a digit are adduction, flexion and abduction. 

 When each digit is supplied with an independent intrinsic muscle for the production of 

 each of these movements the three sets of muscles assume the trilaminar arrangement. 

 Such a hand or foot is typical in so far as it presents an arrangement to which all other 

 hands or feet can be referred. 



The centre for the movements of adduction and abduction. — A decided tendency is 

 exhibited throughout Mammalia generally for the adductors and abductors of the toes to 

 arrange themselves with reference to the medius or middle digit. It thus comes about 

 that the adductor medii disappears and two dorsal interossei or abductors are allotted to 

 the medius, one for each side. There are a great number of exceptions to this rule. In 

 Echidna adduction takes place towards the interval between the hallux and index ; in the 

 Ornithorhynchus, Cuscus, and Phalangista vulpina (?) towards the index ; in the Koala, 

 Phalangista vulpina (?), and Tamandua towards the annular digit. In other cases, where 

 only one or perhaps two adductors are present, it is impossible to make out the exact 

 point, although it may be presumed to be the central line of the foot. Exceptions in the 

 case of the abductors are of rarer occurence. We may quote, however, Man, the Gorilla, 

 and the Tamandua as marked instances in which the centre of abduction is the index, and 

 the Lemur, as an instance in which the annular digit constitutes the centre. It sometimes 

 happens that the centre of adduction does not correspond with that of abduction. The 

 Tamandua and the Lemur are examples of this. In the former there is no adductor of the 

 medius ; but the line of origin of the three remaining adductors is over the metatarsal of 

 the annularis, and therefore we conclude that they operate towards this digit, and that 

 the adductor indicis in acting upon the index has an indirect influence upon the medius 

 as well. The abductors are, on the other hand, distinctly arranged with reference to the 

 index — two being appropriated to this digit, and the others being inserted into the outer 

 sides of the medius, annularis, and minimus, and into the inner side of the hallux 

 respectively. In the Lemur, whilst abduction takes place with reference to the 

 annularis, adduction is effected with reference to the medius. The adductor annularis, 

 however, which is rendered useless by the fact that this digit is supplied by two dorsal 

 interossei is evidently fast disappearing, indeed it is not described by Murie and Mivart. 



In the foot of Man, the Gorilla, &c. , certain of the flexores breves by the loss of one 

 of their heads became adductors, and thus placed themselves in antagonism to the 

 abductors. In tables A. and C. the centres of adduction and abduction are indicated as 

 far as it is possible to do so by the introduction of an asterisk (*). 



