PHYSIOLOGY OF MOVEMENT. 739 



simultaneously. A running jump ma}- be made higher or broader than 

 a standing jump from the additional impetus acquired through inertia. 



4. The Gaits of the Horse. — Although the acts of locomotion in 

 quadrupeds are much more complicated than in man, they may be all 

 reduced to variations of three main types — the walk, the trot, and the 

 gallop.* Since in quadrupeds the centre of gravity lies in front of the 

 centre of the trunk, it can only be advanced by power acting from the hind 

 extremities, the fore legs being concerned simply in supporting the trunk. 

 In the horse the posterior extremities are especially fitted for this act by 

 the angular character of their joints, so that in the action of the extensor 

 muscles the hind legs become considerably longer, and the foot remain- 

 ing in contact with the ground, through the contraction of the extensors 

 the result must be to advance the upper extremity of the leg forward ; 

 and the greater the angles of the posterior extremity, the farther the 

 trunk will be advanced in the straightening of the hind leg. The 

 extremity, which in the commencement of the extension of the hind 

 leg was behind, will now be advanced so as to support the trunk, exactly 

 as has been found to be the case with the swinging leg of the walking 

 man, which, immediately after the impulse of the active leg, swings for- 

 ward, and then on its part assumes the role of the propulsive leg, while 

 the previously active leg now becomes passive. The force of the shock 

 communicated to the trunk by the hind legs will be somewhat diminished 

 by the oblique insertions of the extremities and b3 T the angles between 

 the single bones of the hind leg, while in the fore extremities the shock 

 of the foot striking the ground will be diminished by the soft parts, 

 muscles and fascia, which connect the shoulder-blade to the trunk. 



Before taking up the consideration of the different gaits of the 

 horse the characters and mode of production of the different movements 

 in the individual limbs first deserve attention, taking up, first, the move- 

 ments of the fore leg and then of the hind leg : — 



The animal being supposed to be in the erect position, before move- 

 ment of the fore leg takes place the body weight is first shifted, through 

 the contraction of the pectoral muscles, aided by the latissimus dorsi, to 

 the opposite extremity. The shoulder being elevated by the rhomboid 

 and trapezius muscles, flexion commences with the contraction of the 

 levator humeri, approximating the humerus to the scapula and diminish- 

 ing the shoulder angle. The principal reduction in the length of the 



* In the account of the different movements in the gaits of the horse the author has 

 mainly followed the analysis published by Bruehmuller, " Lehrbuch der Physiologie," Oes- 

 ierrelcher Vierteljahresschrift filr Veterinarkunde , liii, 1880, pp. 97-130, based on instan- 

 taneous photography. Acknowledgment is also due to Colin ," Traite de Physiologie 

 Comparee;" Munk, " Physiologie des Menschen und der Saugethiere ;" Boehm, Archiv 

 fur WissenschaftlicTu und Praktische Thierheilkunde, Bd. xiii and xiv ; and Schmidt- 

 -Mfllheim, " Physiologie der Haussaugethiere." 



