ANATOMY OF A BIRD. 247 



outer side. In contracting, it would press on the optic nerve, were it not for the other one, which, 

 however, is so disposed that by its contraction it draws away the tendon of the pyramidalis muscle 

 from pressing on the nerve. As in ourselves, there are six special muscles for moving the orbit or 

 ball of the eye, but the one which in man is well enough known as the trochlear, has no pulley- 

 arrangements in birds. Lachrymal glands are present. 



With regard to the organ of hearing, one particular part, which in man is in the form of a snail's 

 s'.iell, and is known as the cochlea, is not coiled into this shape in birds, being very slightly bent, though 

 holding in other respects the same general relations. Nor is there any external ear, as in mammals, 

 far collecting the waves of sound ; there is, however, in the nocturnal birds of prey a crescent-shaped 

 valve on which are set tufts of short feathers, and it is possible that this may aid in hearing. Nor, 

 again, are there in the interior of the ear those three small bones, which are known generally as the 

 auditory ossicles ; of the two that are absent, one is thought by many anatomists to be represented by the 

 quadrate bone, which, as has already been mentioned, connects the lower jaw of the bird with the 

 skull. The single bone which is present, and which is, perhaps, most generally known as the 

 " columella," is connected by two or three cartilaginous processes with the drum of the ear, and by the 

 other end at which it has a small oval plate with the more internal parts of the organ of hearing. 

 In man there is a curious arrangement of rods, which vary in so remarkable a way as to have led to 

 the supposition that each was adapted to a distinct note ; these rods, which constitute the organ of Corti, 

 are not present in birds, affording thereby a striking example of the law that physiological inferences 

 are often well examined by the aid of comparative anatomy, no physiologist being hardy enough to 

 deny to birds the power of appreciating those delicate modulations of sound which go to make up the 

 chief charm of music. With regard to the organ of smell, it is only necessary to note the absence of 

 those muscles by which, in man and other mammals, the nostrils are contracted or dilated. 



The first point which attracts us on examining the digestive tract of birds is the absence 

 of lips and of teeth ; but with regard to these latter we must note that it is a character which 

 has only become distinct since the time when birds were first developed. This statement is borne 

 out by two series of facts, each taken from one of the two great aids to a correct apprehension of 

 fie real importance of structural characters that is, from embryology, or the study of the developing 

 individual ; and from palaeontology, or the natural history of the past. The young of certain Parrots 

 have been observed to possess, at an early stage of their development, those uprisings on the mucous 

 membrane of the jaw which go by the name of " dental papillae," and these papillae have been 

 seen to be covered with a cap of dentine. On the other hand, the researches of Owen and of some 

 American palaeontologists have brought to light bird-like forms which were provided with teeth 

 (Odontornithes : Ichthyornis, ffesperornis). 



The beak, or horny covering of the jaws, varies very greatly in form, and in the degree of its 

 sensibility. This tactile sense is dependent on the extent to which the beak is supplied by nerves 

 (from the fifth cerebral nerve). In the Woodpecker, for example, there is a large branch ex- 

 tending along the inside of the lower jaw, which, as it approaches the extremity, breaks up into 

 finer nerves that perforate the bone by a mimber of small canals and so give to the beak a power of 

 discovering what lies hid in the crevices of the wood and under the bark. Being an external structure, 

 the beak is naturally adapted to the habits of its possessors, so that it may be hooked, as in many 

 flesh-eating forms, or trenchant, and fit to cut and break, or provided with transversely-set fine plates 

 by which the water taken in with the food can be filtered off, or provided with bristles, the better 

 to hold a living prey. Finally, in many cases the hardness of the bill is made up for by a patch of 

 naked skin at the base of the upper mandibles, which is known by the name of the " cere " and seems 

 to have a tactile function. 



In many birds, the tongue is either feebly developed, or is encased in horn, so that it can hardly 

 be as useful an organ of taste as is our tongue : in the Pelicans it is obsolete. In some, however, as 

 in the Woodpecker, the tongue is a very powerful seizing organ, as it is protruded with great rapidity 

 by means of a special muscle, and is well provided with a sticky secretion, which is given off from a 

 large gland (the sub-lingual), which, lying below the nmscle above referred to, is compressed when 

 this muscle contracts ; so that in the Woodpecker, just as in the mammal called the Great Ant-eater 

 (Myrmecophaga^), the insect prey is easily captured. 



