THE BLOOD SYSTEM 25 



Arising from the vcntricU' of tho heart and passing oblicjuely 

 aeross the aiirieles is the hir^e vessel, tnincus (irteriosus, referred 

 to above. Note the divisions of the truncus arteriosus into riglit 

 and left branches, each of which again divides into three arches. 

 l']ach of these arches supplies one of the regions mentioned above. 

 In order to gain a clear view of these three arches and the various 

 arteries arising from them, it will be necessary very carefully to 

 remove the veins and other structures which overlie them. 

 Xcvcr take hold of a blood vesacl itself with forceps. Handle it 

 by seizing the supporting or associated tissues. Begin at the trun- 

 cus arteriosus and trace out the arches and arteries of the left 

 side of the animal only, since the two sides are alike, reserving 

 the right side for the study of the veins. 



The branches of the main arches and their subdivisions are 

 as follows: 



1. The carotid arch (Fig. 1, I) is the most anterior of the three. 

 It extends dorsad around the side of the esophagus and soon 

 divides into two branches. A small, vascular swelling, the 

 carotid gland, surrounds the base of these branches and often 

 prevents the injecting mass from filling the vessels. The two 

 branches are: 



a. The external carotid artery, a small vessel supplying the 

 tongue and floor of the mouth; 



b. The internal carotid artery extending around the (psophagus 

 to its dorsal side, then cephalad into the roof of the mouth along 

 the base of the skull. Ix)cate its origin from the carotid arch. 

 Cut the lower jaw at the angle of the mouth. Peel the lining 

 from the roof of the mouth and note its distribution in this 

 region. 



Which general region of the body is supplied by the carotid 

 arch? 



2. The pulmo-cutaneous arch (Fig. 1, III) is the third and most 

 caudal of the three arches. It will be described and dissected 

 at this point, since its branches are likely to be destroyed in the 

 dissection of the middle arch. The pulmo-cutaneous arch divides 

 at about the level of the carotid gland into (a) the cutaneous 

 artery supplying the skin. Its course can be readily traced. Slit 

 the skin along the middorsal line and turn it back until the 

 shoulder region is visible. The course ami distribution of the 

 cutaneous artery may then be seen, (b) The pulmonary artery, 

 following a somewhat wavy course along the outer side of the 



