6 Dr. Mac Culloch on the Means 



positions and actions of the last pair of flexor and extensor 

 muscles ; and thus it has been been attempted to explain it. It 

 is obvious, however, on a moment's consideration, that no actions 

 of these muscles, however powerful, could alone produce the 

 consequence in question ; as they could have no further effect 

 than that of fixing that part of the fifth phalanx, which is below 

 the suture, more firmly on the body. To explain the mode in 

 which their action is rendered efficient towards the separation of 

 the limb, it is necessary to describe the forms and relations of the 

 fifth and sixth phalanges more particularly ; as, in these, the true 

 solution of the difficulty will be found. It is from inattention to 

 these important circumstances, that this process has appeared so 

 mysterious ; and assuredly it is difficult, on a first view of the 

 operation as performed by the animal, to witness it, without sur- 

 prise at the facility with which it is effected, and at the apparent 

 inadequacy of any conceivable means for producing the separa- 

 tion. The obvious effect of muscular action, is to approximate 

 the insertions of the muscles ; yet here it appears to act as if it 

 was to separate them. 



As it is impossible to render this structure intelligible without 

 drawings, some sketches are added for that purpose; and, as 

 being more obvious in the common crab, (Cancer pagurus,) they 

 are taken from that species. They are limited to the fourth, 

 fifth, and sixth phalanges ; being the only ones required for 

 illustrating the subject ; and these are numbered, so as to cor- 

 respond to Fig. 1, Plate I. The place of the suture is indicated 

 in these sketches, as it is not so defined externally as in the spider 

 crab. In Plate I. Fig. 5, is an outside view of part of the limb in 

 a state of flexion ; the state of extension being performed by the 

 approximation of the points A, B, until they meet, so as that the 

 indicating lines coincide in a common line C. In Fig. 2, Plate J, 

 the phalanx is in a half extended state, and is further so turned, 

 as to shew more distinctly the forms of tke two phalanges at A 

 and B ; while, in Fig. 3, Plate I, the extension being completed, 

 the points A, B, are' brought into absolute contact at G. Another 

 view of the meeting of those points when in a state of extension 



