XIIL] THE FROG. 203 



b. The pulsation of the heart. This should .be stu- 

 died in a Frog rendered insensible by chloroform 

 or by being pithed; though the latter operation 

 causes such dilatation of the vessels that little or 

 no blood may afterwards flow through the heart, 

 yet the organ goes on beating. 



a. Watch the movement carefully; it is a regu- 

 larly alternating series of contractions and dila- 

 tations. 



b. It will be seen that the two auricles contract 

 together; immediately after them, the ventricle; 

 and then, instantly, the bulbus arteriosus. 



c. Raise the ventricle so as to see the venous 

 sinus; note that it contracts immediately before 

 the auricles. 



6. The parts exposed by the preceding dissections 

 (B. i. 2). 



Draw them carefully without disturbing them. 



a. The throat-muscles: through the broad thin mus- 

 cle in front (mylo-hyoid) is seen the hypoglossal 

 nerve. 



b. The larynx: forming a hard prominence in the 

 middle line, just in front of the aortic arches. 



c. The heart and aortic arches (see B. 5. i.) : the 

 three terminal branches of the latter, viz. 



a. The carotid trunk; the anterior division; end- 

 in a small reddish body (the carotid gland}. 



/?. The systemic aortic arch, 

 y. The pulmo-cutaneous artery: the hindmost 

 branch, 



