92 ELEMENTARY BIOLOGY. [CHAP. 



a. The aortic and carotid arches ; from its an- 

 terior end, immediately in front of the longi- 

 tudinal septum, the calibre of the aortic arch 

 being much the greater of the two. 



y8. The pulmonary arches ; the pair arise by a 

 single aperture lying immediately to the 

 animal's left of the anterior end of the longi- 

 tudinal valve. 



iii. A companion dissection may be profitably made 

 from the left side, for general comparison with the 

 foregoing, especially with respect to the relations 

 of the auriculo-ventricular valves and inter- auricular 

 septum. 



12. The circulation of the blood in the web of the foot. 



i. Get a piece of thin board, about 5 inches long and 

 2\ broad ; in the middle of one end of it cut a 

 V-shaped notch about the size of the expanded 

 web : place the frog on the board, belly down- 

 wards, and fix it by passing round it two or three 

 turns of tape : next tie threads round the toes of 

 one foot, and by means of them spread out the 

 web over the notch, taking great care that it is 

 only very lightly stretched. The animal should be 

 kept moist by spreading a piece of wet blotting- 

 paper over its back. 



ii. Examine the web with i inch obj.: Note 



a. The black pigment- cells in the skin ; sometimes 

 irregularly branched ; sometimes more compact. 



b. The close network of blood-vessels, lying deeper 

 than the pigment-bearing layer. 



