558 PHILOSOPHICAL TRANSACTIONS. [aNNO 1795. 



becomes altered, its axis being elongated. This construction and consequent 

 convexity of the cornea, must render small objects near the animal very distinct. 

 On these muscles relaxing, the elasticity of the sclerotic coat will restore the 

 cornea to its original flatness; it thus becomes fitted for viewing objects placed 

 at a greater distance from the eye, and this will be in proportion to the degree 

 of relaxation. 



There seems to exist in nature an economy of motion, to prevent fatigue and 

 exhaustion of the animal powers, by continued voluntary muscular motion. If 

 2 o[)posite actions of the same frequency occur in 2 muscles, the one being an- 

 tagonist to the other, the action of one ceasing, the action of the other must 

 take place previously to further motion of the part; for instance, on the biceps 

 flexor of the arm acting, the arm will be bent, but on discontinuing its action 

 the arm will remain in the same state, unless it was straightened by the action of 

 the biceps extensor its antagonist; but where one action in a part is required to 

 take place almost ronstantly, and the opposite action but seldom, to save the 

 animal from fatigue, necessarily induced by muscular contraction, she gives an 

 elastic ligament, which from its elasticity may be said to be in continual motion 

 without exhausting the animal. Thus when the opposite action which is of less 

 frequent occurrence is required, it is performed by overcoming the resistance, or 

 elasticity of this elastic ligament, which on the muscle giving over its action 

 again, resumes its former state. The elastic cartilages of the ribs performing in 

 some degree the function of a muscle, are of use in respiration; likewise the 

 elastic ligaments which support the claws of all the feline genus, keeping them 

 from friction against the ground. These claws at the volition of the animal, by 

 muscles appropriated for that purpose, are brought into action or extended. 

 From the above-mentioned structure, the same thing appears to take place in the 

 eyes of animals. When an animal is desirous of seeing minute objects, the 

 recti muscles act, and thus, by rendering the eye more convex enlarge the angle 

 under which the object is seen. How necessary is this structure to these animals 

 in particular; for without it a bird would be continually exposed to have its head 

 dashed against a tree when flying in a thick forest, its motions being too rapid 

 for the common structure of the eye. The eagle, when soaring high in the air, 

 observes small objects on the earth below him, inconceivable to us, and darts 

 upon them instantaneously. Here we must allow that there must be an extra- 

 ordinary alteration in the focus of this eye in almost an instant of time. How 

 could this be performed unless the animal had this apparatus? The eyes of qua- 

 drupeds, as I shall afterwards show, can perform this alteration ; though not in 

 the same degree, as it is not necessary, their modes of life being difierent. A 

 swallow sailing through the air pursues a gnat or small fly to almost certain de- 

 struction. This apparatus is very distinct in all these birds. Whenever we find 



