221 



almost entirely upon the form of the articulating surfaces, which is 

 such that the tarsus is at the greatest distance from the centre of 

 motion (i. e., the upper attachment of the lateral ligament) when it is 

 midway between the two extreme positions, which is at an angle of 60° 

 with the position of the greatest extension, and of 40° with that of the 

 greatest flexion. Passing beyond the point of the greatest tension 

 forwards, it suddenly slips upon a curve of shorter radius, when it 

 meets with less resistance, and as the elasticity of the ligament now 

 comes into action, the tarsus is brought suddenly nearer to the centre 

 of motion, and thus moves forward with a jerk. 



Moving backwards, after passing the position of the greatest ten- 

 sion, it comes upon a curve of larger radius, but at the same time 

 nearer to the attachment of the ligament ; and as here it meets with 

 less resistance again, it moves suddenly backwards, until it is nearly in 

 a straight line with the tibia. In this last position its further motion 

 is arrested, in consequence of the curve of the bone being of a large 

 radius, and thus carrying the tarsus further from the centre, and also 

 in consequence of striking against a projection of cartilage on the 

 lower end of the tibia. 



The ligaments which become tense during these movements are the 

 long internal and external lateral ones. In front of the inner one, 

 and nearly parallel to it, is a short ligament, which becomes stretched 

 only when the tarsus reaches its greatest extension. On the outer 

 side, the long ligament is crossed by the shorter one, this last serving 

 too to resist extension beyond a straight line. The relative stretching 

 of the inner and outer long ligaments may be learned from the follow- 

 ing measurements : — 



Inner, Outer. 



When tarsus is extended . . . 8.60 in. 3.05 in. 



" " half-flexed . . . 8.70 " 2.97 " 



" " flexed . . . 7.95 " 2.75 " 



Whole amount of stretching . . 0.75" 0.30" 



From this it will be seen that the stretching is confined mostly to 

 the inner long ligament, which increases in length 0.75 of an inch, 

 while the outer is stretched only 0.30 of an inch. 



The ligaments above described are composed of white fibrous tissue, 

 but which seems to be endowed with a greater degree of elasticity than 

 is usually found in this substance. It is supposed by most anatomists 

 to be wholly destitute of this property, and is doubtless as inelastic 

 in most cases as it is possible for any animal tissue to be. 



The Corresponding Secretary read the following letters re- 

 ceived since the last announcement, viz. : — 



From Samuel R. Carter, Paris Hill, Maine, March 16, 1863, in 



