a correct System of Muscular Homologies. 315 



nerally exhibit an individualitj of action separate from the 

 others : these are the inner and the outer. For these we have 

 what at first sight appear as separate special muscles often 

 present ; but, as Meckel remarked, these can be resolved into 

 the ordinary and typical series which we have developed for 

 the others, only in a position of greater specialization. 



Discarding for the present the longer digital muscles, we 

 can resolve the muscles of the pes and manus into the fol- 

 lowing : — 



1. A short extensor for the fingers or toes. 



2. A short flexor for the fingers or toes. 



3. A complete series of palmar interossei. 



4. A complete series of dorsal interossei. 



The first of these muscles is developed on the dorsum of the 

 human foot, and is there attached to the four inner toes ; only, 

 however, in the case of the great toe is its typical insertion 

 into the first phalanx preserved, as in the three other digits its 

 tendon is confluent with that of the long extensor. It is not 

 usually inserted into the little toe ; but I have once seen this 

 muscle in man sending a fifth tendon to that organ. In the 

 manus of man a corresponding muscle occurs as an anomaly 

 occasionally, and as such has been described by Mr. Wood 

 (Proc. Koyal Soc. 1865, p. 382) and by myself (Proc. Koyal 

 Irish Acad. April 1867) ; when present, it is often inserted, 

 fleshy or tendinous, not directly into the fingers, but into the 

 extensor longus tendons. This muscle is of rare occurrence in 

 the nonnal anatomy of lower animals ; to my knowledge it is 

 only described as present in the Bradyims tridactyla^ according 

 to Meckel; and the same writer describes a corresponding 

 muscle arising from the lower end of the ulna in the two-toed 

 Anteater. The comparative anatomy of this muscle in the pes 

 exhibits but little variety ; it is present in Ornithorliynchus^ 

 Hyrax^ Myrmecophaga, Macropus, Arctomys^ Bear, Nasua, 

 Ptevopus^ Rhinolophus^ Ste-iiops, Macacus, Ateles, Cebus, Calli- 

 thrixj Cercopithecus, Ai, Hystrix, and many other animals, in 

 different degrees of perfection, but never varying to any great 

 extent from its usual place. 



The second of these muscles may be looked upon as the 

 antithesis of the short extensor : it is the short flexor of the 

 toes and fingers. If we compare the actions of flexion and 

 extension of the digits, we shall find at once that, usually, the 

 latter is provided for by the great development of large and 

 long muscles arising in the forearm. On account of this the 

 flexor of the first phalanx is not usually needed in this its 

 primary capacity ; and consequently we find it split up or mo- 



23* 



