MYOLOGY. 167 



what by pressure from above, and combining to secure elasticity of the parts at rest 

 or in motion. The inner arch is formed by the os calcis, astragalus, scaphoid, and 

 three cuneiform bones, together with the three inner toes ; the head of the astragalus 

 being the key-stone, and being supported in its position, not by immoveable piers, 

 as would be the case in an arch of mason-work, but by them in connection with the 

 inferior calcaneo-scaphoid ligament. The outer arch is formed by the os calcis, 

 cuboid bone, and two outer toes, and is supported by the strong inferior calcaneo- 

 cuboid ligaments. Thus the calcaneo-scaphoid and calcaneo-cuboid ligaments are 

 stretched by the whole weight of the body bearing down upon the arch, and 

 prevent the too great flattening of the instep ; an action in which they are assisted, 

 however, by the plantar aponeurosis. The transverse arching of the foot is most 

 marked in the line of tarso-metatarsal articulations, and is maintained by the wedge- 

 shape of the bones and by inferior ligaments. The weight of the body, falling upon 

 the balls of the toes when the heel is raised, tends to spread out the metatarsal bones 

 at their distal extremities, and to flatten the transverse arch, which recovers its 

 position when the pressure is removed. Between the astragalus and the calcaneum 

 only one kind of motion is possible, the centre of which is the interosseous 

 astragalo-calcaneal ligament, and is of such a nature, that when the posterior part of 

 the os calcis slides inwards and upwards beneath the astragalus, its cuboid extremity 

 moves downwards and outwards. A certain amount of gliding movement is also 

 allowed between the tarsal and metatarsal bones, and that most considerably between 

 the cuboid bone and outer toes. Thus it happens that if the foot is flexed against the 

 leg, and then is gradually extended by force applied to the toes, as by the action of 

 the flexores digitorum muscles, the first part of the movement is accomplished at the 

 ankle, and consists of extension, with only a slight inward turning of the toes; 

 further extension is accomplished by movement between the calcaneum and astra- 

 galus, and is accompanied by depression of the outer edge of the foot ; and after that 

 a little more extension, accompanied by more considerable depression of the outer 

 edge of the foot, is effected at the tarso-metatarsal articulations. The direction of the 

 movement of extension of the toes at the metatarsal-phalangeal articulations is 

 upwards and outwards, so that although the great toe is in a line with the inner edge 

 of the foot when resting on the ground, it is no longer so when over-extended, as, for 

 instance, ^Yllen supported by a shoe. 



SECTION III. MYOLOQY. 



UNDER the title MYOLOGY will be brought the systematic description of 

 the form, position, and relations of the muscles of the body with their 

 tendons, and in the same section it will be convenient to include that of 

 the fasciae and aponeuroses. 



As many of the muscles consist of tendinous as well as muscular parts, 

 the description of the tendons and their sheaths necessarily forms a part of 

 that of the muscles ; and farther, as it is impossible to separate the exami- 

 nation of the attachmemts of a muscle, or of groups of muscles, from the 

 study of the motions produced by them, the statement of the action of the 

 muscles will accompany their anatomical description. 



In the description of the muscles, it is customary to state the attach- 

 ments of their opposite ends under the names of origin and insertion ; the 

 first term being usually applied to the more fixed or proximal, and the 

 second to the more movable or remote attachment : but it is to be observed, 

 that it is sometimes difficult to lay down a rule for the correct use of these 

 terms, and that in almost all instances it is of importance to consider the 

 action of a muscle as it may affect the motions of the parts attached to 

 both of its extremities. 



As the muscles belong to the double symmetrical parts, it will be under- 



