6 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 123 



spasmodic and unpredictable motions of the jaws caused by contrac- 

 tions of other muscles. Although 20 volts probably did not pro- 

 duce maximum contraction of the depressor mandibulae, it produced 

 consistent jaw motions that could be safely attributed to con- 

 traction of the muscle under study. 



The maximum number of consecutive stimulations, each of which 

 lasted about two seconds, was 18 for a grosbeak and 17 for a chicken, 

 during which the depressor muscles showed no signs of fatigue. 

 Despite extensive severing of other jaw muscles the birds continued 

 to respond well. All birds were killed while still under anaesthesia. 

 The nature and extent of the operative procedures were then checked 

 under a dissecting microscope immediately following each experi- 

 mental series. 



Just before stimulation of the muscles and again during stimulation 

 and steady tetanic contraction, side view photographs of the jaws 

 were taken. The two photographs from each experimental set were 

 projected and traced, one superimposed on the other. I measured 

 the motion of each jaw as degrees of arc between the resting position 

 and the stabilized position during muscle contraction. The lower 

 jaw has a longer radius of motion than the upper jaw because its 

 fulcrum of rotation lies well behind that of the upper jaw. For this 

 reason, the tip of the lower jaw travels farther than that of the upper 

 jaw for the same number of degrees of movement. One therefore can- 

 not obtain a meaningful measure of total gape simply by adding the 

 degree motion of the two jaws, but differences within each jaw can 

 be compared directly. 



Results. — Figure 4 presents the degree of motion of each jaw in 

 bar graphs, whereas figure 5 shows several tracings of the jaws to give 

 the reader a better impression of the actual amount of jaw motion. 

 I must emphasize that throughout the experiments the depressors 

 probably never were contracted maximally and the major protractor 

 muscle of the upper jaw (protractor quadrati et pterygoidei) did not 

 contribute to the observed jaw motion. As stated before, the evening 

 grosbeak lacks a well-defined postorbital ligament, whereas the do- 

 mestic chicken has a stout one. 



It is clearly indicated in figure 4 that muscle contractions in the 

 chicken produced repeatable results during experimentation with each 

 bird (see A 1 and 2, 4 and 7; C 1 and 2; D 4 and 6, 7 and 9) and com- 

 parable results between different birds treated on different days (A 1, 

 B 1, C 1, D 5). 



The results of the experiments may be summarized as follows: 

 (1) the depressor mandibulae caused protraction (raising) of the upper 

 jaw and depression of the lower jaw in the domestic chicken and 

 evening grosbeak; (2) the muscle caused opening of both jaws in the 



