24f> NATURAL HISTORY. 



provided with those convolutions which, iu mammals, seem to increase in complexity of character as 

 the animal rises in the scale of intelligence. The cerebrum does not cover the cerebellum. Small as 

 is the brain, of birds, it is found that, in many, its weight is, as compared with that of the body, 

 much greater than it is in man. 



With reference to the spinal cord, or the continuation of the central part of the nervous system 

 through the vertebral column, it is only necessary to remark that it is much increased in width at the 

 two regions, in which the nerves for the fore and hind limbs are respectively given off; that there is a 

 narrow canal running along its centre, and that at the lower end there is a large space. In regard 

 to the cerebral nerves, those for the eyes are of great size. 



Coming now to consider the organs of the senses, and beginning with the eye, it is interesting to 

 note that there are no blind birds, and, indeed, the eyes are of a large size as compared with the 

 brain. They are generally placed at the sides, though the nocturnal birds of prey (in which they are 

 directed forwards) are an exception to this rule. It is in very rare cases that eyelashes are present, 

 and although they seem to exist in the group just mentioned, it is probably more correct to look upon 

 them as slightly modified feathers. 



If the eye be regarded as having on its front face, a part which would, if completed, form part of 

 a smaller circle than the rest of the eye, it is clear that this cornea, or front part, would be more 

 convex than the rest, and that it would consequently be a " more powerful glass," inasmuch as it 

 would exert a greater bending (refracting) influence on the rays of light which pass through it, while, 

 further, it is clear that the more convex it is the better "glass" would it be. Now this is just what 

 happens in birds : the cornea is very convex ; in addition to this, the long axis of the eye, on the length 

 of which it seems that, in many cases, the condition known as that of being " short-sighted " depends, 

 is very long in some birds, and notably in the Owls. 



The eye is covered in by a firm and strong membrane, which is known as the " sclerotic;" this, 

 in its front part, develops a number of bony plates ; of these there may be as many as twenty, and 

 they are capable of a certain amount of free movement on one another. What is known as the power 

 of accommodation depends upon the extent to which the front face of the somewhat lens-shaped 

 body which helps to separate the eye into two chambers is capable of being rendered more or less 

 flat ; this front face is covered by a membrane which is found to be more or less taut, according to the 

 state of contraction of the muscles (ciliary muscles) connected with it. A very little reflection is sufficient 

 to show that a swiftly moving animal has the focus of its eye, or the point at which clear vision is alone 

 possible, changed much more rapidly than an animal which moves more slowly. So much on the one 

 side. On the other, it is to be observed that muscles vary in structure ; they are either " smooth " or 



" striated," and it is the latter that contract the more rapidly. Putting 

 these two series of observations together, it is easy to arrive at the result 

 that a bird should have striated muscular fibre in its ciliary muscles, and a 

 more slowly moving animal like man, smooth muscular fibres ; and this we 

 find to be the case ! The iris is an arrangement by which the quantity of 

 light admitted into the eye is enabled to be varied, and the small hole in 

 the centre, through which the rays of light pass, is known as the pupil; 

 this is always rounded in birds, and is never elongated as it is in some 

 mammals the Cats, for example.* 



But the most peculiar arrangement in the bird's eye is the presence, pro- 



SECTION OF THE EYE , . . , .. , . , , . .. , i_ i 11 J 1 



OF THE COMMON BUZZARD. J ectin g mto hinder chamber, of a membrane in which run blood-vessels ; 



(After MacgiUivray.) this, which is known as the pecten (comb), or marsuj)ium (pouch), enters the 



^pi^H^T^^fciiiarT^lr^Td! vitreous humour, which tills up this hinder chamber by the same cleft as 



bi^xeSe^f'pe&n. *' w the optic nerve. It is folded, and is generally of a quadrangular shape; 



it is not found in the eye of the Wingless Bird of New Zealand (Apteryx). 



A third eyelid is well developed in this class ; it is an elastic membrane (membrana nictitans, 

 or winking membrane), which has not, like the other two, a vertical movement, but is drawn 

 obliquely over the eye from the inner to the outer side. This movement is effected by two special 

 muscles, one of which arises on the inside, and below the eyeball, and has therefore to pass over to the 



* Compare Vol. I., p. 213. 



