BIRDS 



149 



going very rapidly, as, if this were not the case, the high 

 temperature of the body could not be maintained. 



Voice. Although the pigeon possesses a larynx, this is not 

 the organ of voice, which consists of a structure known as the 

 song-box or syrinx ', formed by the modification of the extreme end 

 of the windpipe and the beginning of the two bronchi to form 

 a small resonant chamber (tympanum] into which an elastic 

 fold projects. The sound is produced by vibration of this fold, 

 and it can be stretched to various extents so as to alter the 

 pitch of the note. 



Organs of Movement. As in the human subject, the white 

 corpuscles of blood and lymph are able to creep about by means 

 of amoeboid movement (p. 39), and the windpipe and certain 

 other structures are lined by ciliated membranes (p. 49); but, 

 as before, the chief movements of the body are brought about 

 by muscular action. The most interesting peculiarities in the 

 muscular system are connected with flight and with perching. 

 The large fleshy mass covering the breast is made up of the 

 pectoral muscles, which are attached to the wings in such a 

 manner as to pull them alternately up and down; but this is 

 not the place to furnish details, which will be given farther on 

 in connection with locomotion. 



The perching mechanism is an arrangement by which the 

 toes can be all brought together at the same time by a pull 

 exerted upon a single tendon connected with two muscles of 

 the leg. When a bird is roosting, the weight of the body bends 

 the limb in such a way as to pull on this tendon and bring all 

 the toes firmly against the branch or other supporting object, 

 the danger of falling off during sleep being thus averted. 



Nervous System (fig. 103). The brain of the pigeon is 

 extremely short and broad, and is distinguished by several 

 peculiarities. The cerebral hemispheres are large and rounded, 

 but are quite smooth externally, and are not, as in mammals, 

 united across the middle line by a fibrous band or corpus 

 callosum (see p. 52). Both these points are related to the 

 comparatively small intelligence of the animal. The organs 

 of smell are not well developed, and therefore, as might be 

 expected, the olfactory lobes are of small size; but on the 

 other hand, in accordance with the unusually acute vision, 

 the optic nerves, optic tracts, and optic lobes are very large. 



