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The Moulting Mechakism in the Heads of Lizakds. 



By H. L. Bbuneb. 



(Abstract.) 

 One of the muscles of this mechanism was described before the Acad- 

 emy of Science several years ago (1). The complete mechanism, which 

 I have recently described in the American Journal of Anatomy (2), in- 

 cludes the following more im])ortant parts : 



1. The veins and blood sinuses of the head. 



2. Special muscles which distend the sinuses and raise the venous 

 blood pressure. One of these muscles (m. constrictor venae jugularis iu- 

 teruae) invests the chief cephalic vein at the point where it passes from 

 the head into the neck. A second muscle (m. protrusor oculi) lies behind 

 the orbit in close relation to the large orbital sinus. 



3. The cardio-accelerator mechanism. During the operation of the 

 moulting mechanism the number of heart-beats increases and a larger 

 amount of blood is sent to the head. 



In the operation of the moulting mechanism two stages occur. The 

 first stage is characterized by contraction of the constrictor muscle and 

 by acceleration of the heart-beat. The veins and sinuses of the head are 

 distended with blood ; the eyes protrude. The second stage is caused by 

 contraction of the protrusor muscle and others which press upon the dis- 

 tended vessels and raise the blood pressure to a higher level. 



The distension of vessels and elevation of blood pressure aid in exuvia- 

 tion by stretching the skiu and by facilitating the processes of metabolism. 

 The moulting mechanism may be set in motion in an experimental way 

 by the application of court plaster, or similar material, to the liead. 



In snakes and turtles the imternal jugular vein is provided with a 

 constrictor muscle, but the protrusor oculi is wanting. The simpler mechan- 

 ism of these forms probably has the same function as the more compli- 

 cated apparatus of the lizards. 



1 Proc. Ind. Acad. Science. 1898. p. 229. 



2 Am. Jour. Anat., Vol. VIT, 1907, pp. 1-117. 



