416 



MOTION. 



Arnott, though it has been erroneously ascribed 

 by Professor Miiller to Weber. Subsequent 

 experiments made by Dr.Todd, Mr. Wortnald, 

 and others, have fully established the mecha- 

 nical influence of the air in keeping the mecha- 

 nism of the joints together. The amount of 

 atmospheric pressure on any joint depends upon 

 the area or surface presented to its influence 

 and the height of the barometer. The forces 

 acting in opposition to the weight of the limb 

 are the pressure of the air, and a force em- 

 ployed by the ligaments and muscles equal to 

 the excess of the weight of the limb, if any, 

 above that of the pressure of the atmosphere. 

 According to Weber the atmospheric pressure 

 on the hip-joint of a man is about 26 pounds. 

 The pressure on the knee-joint is estimated by 

 Dr- Arnott at 60 pounds. This estimate agrees 

 with our measurement of the area of the surface 

 of the knee-joint in an adult female, but is too 

 small for an adult male which is about 90 

 pounds. In the Elephant and Megatherium, 

 the pressure of the air upon the joints is 

 greater in proportion to the increased bulk and 

 weight of their limbs. In the latter the area 

 of the plane bounded by the edge of the coty- 

 loid cavity, is an ellipse whose diameters are 

 seven and eight inches, and therefore present 

 a plane surface exceeding 43.9824 square inches, 

 which, being multiplied by fifteen, with the 

 barometerat 30inches,wil!be=43.9824 X 15, 

 or about six hundred and sixty pounds pres- 

 sure upon the cotyloid joint of this greatest 

 of terrestrial Mammalia. 



Muscles. The amount and effects of mus- 

 cular contraction, the absolute and relative 

 power of muscles in reference to their length, 

 mass, and obliquity of direction, and the ex^ 

 penditure of their power to support given 

 weights, will now be either simply noticed or 

 briefly estimated. 



In the application of muscles to the purposes 

 of locomotion we find them so arranged as to 

 produce great velocity, and at the same time to 

 admit a great extent of motion, still preserving 

 the beauty of proportion. These objects are 

 obtained, 1st, from the oblique directions of 

 their fibres towards the tendons ; 2d, from the 

 obliquity of the direction of the tendon to the 

 bones on which they act ; 3d, from the proxi- 

 mity of their points of insertion to the articu- 

 lations of the bones, or axes of motion in the 

 joints. 



The muscles are capable of contracting (ac- 

 cording to the researches of MM. Prevost and 

 Dumas) about -^ or nearly one-fourth their 

 whole length, which, owing to the circumstances 

 just mentioned, is sufficient to produce all the 

 positions and motions observed in animals. 



It has been already determined by experi- 

 ment that the volumes of muscles do not alter 

 by contraction, their thickness only increasing 

 as they decrease in length, and vice versa. 



The comparative power of muscles in the 

 same animal, according to Borelli, may be 

 thus estimated: 



When two muscles are composed of an equal 

 number of fibres, or are of equal thickness but 

 of unequal lengths, they will suspend equal 



weights, but their motor powers and the height 

 to which they are capable of raising the weights 

 will be as the lengths of the muscles. 



2d. When the lengths of the muscles are 

 equal and their thicknesses unequal, their rela- 

 tive powers will depend upon their thicknesses, 

 but they will raise weights to equal heights. 



3d. When, the lengths and thicknesses are 

 unequal, the weights they will raise will depend 

 on their thicknesses, and the heights to which 

 they will raise them will be as their lengths. 



When the fleshy fibres of a muscle lie pa- 

 rallel to the tendon, the space through which 

 they will draw it equals the contraction of the 

 fleshy fibres ; tut when they are inserted ob- 

 liquely into the tendon, the space through 

 which they will draw it will vary with the in- 

 clination. 



Thus, let two equal fleshy fibres, AC, B C, 

 (Jig. 218) similarly situated with respect to the 

 tendon C D, be inserted obliquely at C, join A B, 

 and produce D C to meet it in P, then D P is 

 perpendicular to A B. Now if the points at A 

 B be considered as fixed, and the angle A C P 

 be such that radius : its sine : : A C : to the 

 length of A C when contracted, then the joint 

 action of the fibres will draw the point C to P. 



Fig. 218. 



For with A B, as centres describe the circular 

 arcs P E, P F, touching each other at P, then 

 it is evident that the point C will, after the 

 contraction of C A, be somewhere in the arc 

 E P, because the radius of E P is the length 

 of A C when contracted ; for a similar reason 

 C will be somewhere in F P ; therefore it 

 will be at P, their point of contact. The 

 same result becomes apparent from the consi- 

 deration that the forces in the direction C A, 

 C B are equivalent to forces in the direction 

 C P, P A and C P, P B respectively, of which 

 the forces in the direction C P are not counter- 

 acted, but gradually diminish and become zero 

 when the fibres are at right angles to their 

 tendon, that is, when C coincides with P. 

 It is here assumed that there is no ob- 

 stacle to the free motion of the tendon in 

 the line C P. 



If the obliquity of the fibres be less than 

 A C P, the arcs will intersect in some point 

 between P and C, and the contractile force 

 will be insufficient to draw C to P. If, on the 

 contrary, the obliquity be greater than the 

 angle A C P, the arcs will not meet, but C 



