44 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



Make an outline drawing of the frog and its organs, and in this put the 

 venous system in its proper relations to the organs. More than one drawing may 

 be necessary. Draw only those vessels which you have found. 



E. THE CIRCULATORY SYSTEM: THE ARTERIAL SYSTEM 



The conus arteriosus starts from the base of the right side of the ventricle, 

 passes obliquely across the auricles, and divides near the anterior border of the 

 auricles into two vessels which turn respectively to the right and left. Each 

 branch is called a truncus arteriosus and each gives rise shortly to three arteries, 

 which are designated as aortic or arterial arches. To find this division into three, 

 carefully pick off the connective tissue, etc., from the surface of one truncus, and 

 follow it away from the heart. The most anterior of the three arches is the 

 carotid arch; the middle one is the systemic arch; the most posterior, the pulmo- 

 cutaneous arch. The pulmo-cutaneous arch generally branches off before the 

 other two (Holmes, pp. 268-70). 



1. The carotid arch. This vessel passes forward and soon divides into two 

 branches, a medial small external carotid, and a lateral larger internal carotid. 

 Just at the place of division, or situated on the internal carotid, is an enlarge- 

 ment, the carotid gland, presenting a blackish appearance owing to the presence 

 of pigment cells. The internal carotid supplies the roof of the mouth, eye, brain, 

 and spinal cord. The external carotid (or lingual) supplies the tongue, thyroid, 

 and various muscles. Trace the branches of the carotid arch as far as practicable. 



2. The pulmo-cutaneous arch. Trace this out from the truncus arteriosus, 

 and note that it soon divides into two branches. One of these, the pulmonary, 

 is short, and runs directly to the lung. The other, the cutaneous, passes outward 

 and forward, crossing the systemic arch, and then by a sharp dorsal turn dis- 

 appears in front of the shoulder. Turn the frog dorsal side up, slit up the skin 

 of the back, and deflect it. Find where the cutaneous artery emerges just in 

 front of the suprascapula, and note its branches on the skin. 



3. The systemic arch. The systemic arch passes laterally and forward, and 

 seems to disappear in the mass of muscles. Dissect away these muscles and 

 follow its course. It runs forward alongside the internal carotid, then bends 

 dorsally and turns backward dorsal to the esophagus. Just after it has made 

 this posterior turn, the systemic arch gives rise to the following branches prac- 

 tically simultaneously: the small esophageal arteries, which supply the esoph- 

 agus; the subclavian, which passes laterally to the foreleg; and the occipito- 

 vertebral, which passes dorsally and promptly divides into an anterior branch, 

 the occipital, and a posterior branch, the vertebral. Both of these can be 

 readily followed in well-injected specimens. Turn the frog dorsal side up, 

 and find the occipital artery emerging just under the anterior border of the 

 suprascapula. Follow it along the head, noting a branch above the eye to the 



