' 



SWIMMING. 



SWIMMING. 



water, and, for the purpOM of progression, the feet, which act as oan> 

 are variously and roost exquisitely contrived. For example, in the 

 Qreb each toe U furnished with a distinct membrane (fg. 11), the 

 margins of which overlap each other; but in the Merganser, and 

 many other aqustio birds, such as the Duck and Goose, the same 

 membrane extends to the three toes. (Pig. 12.) In swimming, the 



Fir 1!. 



Eider Duck. 



effective stroke is produced by the feet alternately ; that is, whilst 

 one foot is pushed backwards the other is drawn forwards. (Fig. IS.) 



Fig. 13. 



In the effective stroke the foot is extended and the toes expanded so 

 as to present the greatest surface possible to the water ; it is then 

 driven backwards with force, the effect of which is to drive the body 

 forwards. In the back stroke the foot is flexed, and the toes are 

 brought together so as to present the least surface to the water, and 

 produce as little action as possible. The back stroke of the leg tends 

 to retard the body, and it is only the difference in the amount of the 

 force of these strokes that is effective. Sometimes both feet are 

 driven backwards and drawn forwards simultaneously, and the body 

 mores by a series of jerks. 



Some of the water-birds make use of their wings as sails ; the swan, 

 for instance, may bo often observed partially to elevate the wings and 

 spread them out to the wind, and thus move by the force of the wind 

 alone, like a sailing vessel. 



It is well known that Fishes reside constantly in water; they are 

 indeed so organised that they can neither live nor move out of it but 

 for a very short time. Tlieir specific gravity is very nearly equal to 

 that of water ; but they are also endowed with the power of varying 

 their specific gravity, so as to raise or lower themselves in the fluid at 

 pleasure. We have seen that the human race can vary the specific 

 gravity of the body, by drawing in and expelling the air from the 

 lungs; but fishes, not having the same kind of respiratory organ*, 

 cannot do so in the mere act of respiration. As a large number of 

 fishes however are obliged to sustain themselves surrounded on all 

 sides by the water, it would require an endless play of muscular force 

 to retain them in such a position, if their ppecific gravity were either 

 greater or less than that of the water. To prevent this continual 

 wast* of vital power, they have been provided with an air-bladder, 



which thoy have the power of distending aud contracting at pleasure : 

 this bladder is placed in the body immediately under the spine, and 

 above the centre of gravity, being the best position to keep the body 

 tteady, and prevent its tuniing over when the air-bladder is distended. 

 When the air-vessel is filled the animal is lighter than water, and It 

 rises ; and when the air is expelled it becomes heavier, and sinks. 

 This hydrostatic apparatus cannot but strike with admiration every 

 one who contemplates the beautiful adaptation of fishes to the end 

 they are destined to serve in animal creation. 



Some fishes, such as the rays and soles, are destitute of a swimming- 

 bladder ; but as they generally reside at the bottom of the sea, they 

 do not require one : when they swim, in order to prevent their sinking, 

 they must use as much extra force as is conferred on other fishes by 

 the air-bladder. The locomotive organs of fishes consist of fins and 

 tail, the former of which are variable in size, number, and direction. 

 The figures of fishes are also various, but in some of them, such as 

 the cod, salmon, and mackerel, the figure is supposed to approximate, 

 more nearly than others, to that which is considered by mathe- 

 maticians to offer the least resistance to their progress in the dense 

 medium they inhabit. In the perch tribe we find the greatest number 

 of fins, being as many as eight ; these are termed the two pectoral, 

 two dorsal, two ventral, one anal, and one caudal. These several 

 names are given in consequence of their relative situations on the 

 body. The pectoral fins are supposed to represent the arms, and the 

 anal fins the legs, of the higher orders of miimnK In the gurnard it 

 may be remarked that the pectoral fins (tiy.ll, a, a') are very Urge, as 



Fig. 11. 



Gurnard. 



are also the dorsal (6 c) ; the caudal (c) increase.? in surface as it 

 recedes from the body. The whole of the fins ore more or less em- 



Fig. 15. 



ployed in certain kinds of movements. 

 In 



n order to ascertain the true use of 

 the fins in swimming, Borrelli having 

 cut off the ventral fins of a living fish, 

 put it back again into the pond. It 

 then rolled from side to side like a 

 drunken man, and could not keep an 

 upright position. When the fish move 

 with great velocity the pectoral fins 

 i are laid close to the body, in order 

 that they may not retard its motion ; 

 and in rapid motion the tail becomes 

 the great propelling organ of motion. 

 We shall therefore now investigate its 

 mode of action. The first movement 

 of a fish from a state of rest is pro- 

 duced by the flexion of the tail (as 

 seen in fig. 15, at a) ; during this move- 

 ment the centre of gravity (c) is drawn 

 slightly backwards. When the tail has 

 arrived at a, it is forcibly extended by 

 its muscles in the direction a i, per- 

 pendicular to its plane : the force of its 

 action upon the water, in a i, is traus- 

 luted to the fish in the direction of 

 t a, causing the centre of gravity (c) 

 to move obliquely forwards, in the 

 direction c h, parallel to i a. The toil 

 having reached the central line c d, 

 its power of urging the I ody forwards 

 not only ceases, but during its flexion 

 on the opposite side in the line a o, it 

 tends to draw the body backwards, 

 in the direction o e. Having reached 

 the point o, it is again rapidly ex- 

 tended in the line o t, causing an 

 impulse pn the centre of gravity in 

 c i, parallel to o t. If the two forces c h and c l> acted simultane- 

 ously, we should obtain the resultant cf; but as they do not, the 

 point (c) will not more exactly in the right line cf, but in a curved 



