406 ' ZOOLOGY 



Bird notes are produced not at the upper end of the trachea 

 as in other vertebrates l^jut near its lower end, where it joins 

 the bronchi. The organ is called the syrinx. Its mode of action 

 is somewhat similar to that of the vocal cords in the larynx of 

 mammals. 



438. The Nervous System and Organs of Special Sense. — 



The cerebral hemispheres are relatively larger than in any of 

 the groups yet studied. Their surface is smooth. The cere- 

 bellum is also large and concentrated chiefly in a central or 

 median lobe. By the growth of these two portions the well- 

 developed optic lobes are crowded into a lateral position. The 

 olfactory lobes are small and the sense of smell is not so acute as 

 in many other vertebrates. The optic lobes and the eyes are 

 well developed and the sense of sight is correspondingly acute. 

 The eye protrudes as a somewhat rounded cone in front. This 

 is supported by a ring of sclerotic (bony) plates. The power of 

 accommodation, that is, of focusing the eye upon objects at 

 different distances, is very great in birds. In addition to the 

 upper and lower lids a transparent fold of the conjunctiva 

 {nictitating membrane) may be drawn over the eye from the inner 

 corner. Hearing is acute, and the condition of the ear is inter- 

 esting chiefly in the facts of the absence of the concha of the ex- 

 ternal ear, and in the presence of a well-developed but uncoiled 

 cochlea in the internal ear. 



Birds are sensitive, easily aroused, active. This nervous 

 activity is correlated with the high temperature, rapid respira- 

 tion, and large consumption of food. 



439. Habits. — None of the animal groups present habits 

 more interesting, more readily studied or more suggestive of 

 the adaptation of structure to the demands of the environment 

 than do the birds. The student will find by observation and 

 by reference to current works on natural history many inter- 

 esting facts in connection with bird-life. Under the suggest- 

 ive studies a partial list of such topics will be found. In the 

 chapter on Adaptations and in the section on the classification 

 of birds (Ch. VIII, and §441) additional facts have been pre- 



