THE SENSES.— BRAIN.— VOICE. IJ 



moment their extensor muscles cease to contract, and it is the 

 continued contraction of these organs that renders standing so 

 fatiguing ; but in the stork and other birds with long legs, it is 

 otherwise ; the lower extremity of the femur has a hollow or 

 excavation which, during the extension of the limb, receives a 

 projection of the tibia, which cannot escape from it without a 

 muscular effort ; the leg once in position, it remains extended, 

 without the animal having any necessity to contract his muscles, 

 and without his experiencing any fatigue. 



20. The sense of touch in birds is necessarily dull, on account 

 of the nature of their integuments. The sense of taste also 

 appears to be obtuse in most of these animals : and in fact, their 

 tongue is almost always hard and horny. In general, the same 

 is true in respect to the sense of smell ; sometimes, however, this 

 sense appears to be very delicate ; for we observe that birds of 

 prey direct themselves by the odour alone to carrion, placed at 

 too great a distance for them to perceive it, notwithstanding the 

 great perfection of their sight. 



21. Generally this last sense is more developed in birds than in 

 all other animals. There is found at the back part of the eye a 

 plaited membrane, called pecteu or marsvpium, which projects 

 from the retina tov/ards the crystalline lens, and seems to be of 

 a nervous nature ; it is also remarked that the anterior face of 

 the ball of the eye is strengthened by a circle of bony pieces, 

 lodged in the thickness of the sclerotica, and besides the two 

 ordinary eyelids, there is always, at the external angle of the eye, 

 a third, named membrana nictitans, (winking membrane,) which 

 may be drawn over the front of this_organ like a curtain. 



22. Birds have not, like most mammals, an external ear; 

 nocturnal birds only, have a large external concha or pavilion, 

 but it is not projecting ; and the opening of the ear is generally 

 concealed by feathers with fringed beards. 



23. The brain is less developed in birds than in most mam- 

 mals, and differs from that of the latter in some important 

 particulars, which v/e cannot enumerate at this time. 



24. Finally, to conclude with the functions of relation, we 

 will add, that in birds the voice is chiefly formed in the inferior 

 larynx, which is situate at the extremity of the trachea, where 

 it bifurcates to form the bronchia. {Plate '^,fig- 2.) In the sing- 

 ing birds this organ is very complicated in its structure ; w^e 



20. Why is the sense of toueh dull in birds ? Why is the sense of taste 

 usually obtuse ? Is the sense of smell very delicate ? 



21. In what particulars do the eyes of birds differ from thoee of mammals ? 



22. How does the organ of hearing in birds differ from that of mammals ? 



23. How does the brain of birds differ from that organ in other animals ? 



24. Where is the organ of voice situated in birds ? 



