10 Dr. Mac Culloch on the Means 



beranecs of the fifth and sixth phalanges at A and B, which, in the act 

 of extension, are to be brought into contact, or to coincide in a common 

 line C. 



Plate II. Fig. 6, is the fifth phalanx, separated from all its con- 

 nexions ; with its flexor and extensor muscles attached at one end, and 

 the lateral muscles, by which it is connected with the fourth phalanx, 

 shewn at the other. The suture is here distinctly seen. This is a joint 

 of the spider crab. 



Plate II. Fig. 7, is a section of the same phalanx, for the purpose of 

 shewing the suture distinctly, together with the insertions of the upper 

 and under muscles. The ends of both those which lie between the fourth 

 and fifth phalanx are shewn; but in consequence of that, only the flexor 

 beneath is visible. It is here distinctly seen, that the shell is reinforced 

 by a ring on each side of the suture ; and that it is not only thinner, but 

 distinctly marked by a line indicating the future division between these. 



Plate II. Fig. 8, represents the dispositions of the fibres at the 

 suture, after the division has taken place. They radiate from a common 

 centre. 



Plate II. Fig. 9, is a kind of diagram, explanatory of the action by 

 which the separation of the fifth phalanx at the suture is effected. The 

 flexor and extensor muscles which produce the disunion of that part, are 

 represented as if the phalanges were transparent, so that their whole 

 .course is seen. The support or resistance which the sixth phalanx gives 

 the fifth, by means of the meeting of the protuberances A and B at C, is 

 also represented. When the limb is thrown into this position of extension 

 by the action of the lower muscles, it is plain that the point C becomes a 

 fulcrum, round which D may be caused to revolve. By means of the 

 action of the extensor, the fifth phalanx is firmly fixed on the sixth, at a 

 point above the suture, and somewhat beneath the upper insertion of that 

 muscle. Thus its principal action becomes that of preserving this posi- 

 tion ; as it has little or no tendency, from the shortness of the lever thus 

 remaining, to bring back the limb into the state of flexion. Hence that 

 action of the flexor, which would otherwise be exerted in retaining the 

 bent position of the limb, is so counteracted that it tends to draw the 

 phalanx asunder at the suture D, where provision is made for that 

 purpose. It is plain that the action of the extensor tends also to produce 

 the same effect ; the great resistance to that flexion, which would other- 

 wise defeat this object, being that of the shoulders of the two phalanges 

 atC. 



However obscure this subject may at first appear to an observer, from 



