232 VERTEBRATE ZOOLOGY 



head ; the temporal artery also goes to the muscles of the head. 

 The carotid artery then ends in a network of small arteries near 

 the base of the jaw, called the carotid plexus. 



Exercise 21. Draw a diagram showing the distribution of the carotid 

 artery and its branches so far as observed. 



The Internal Structure of the Heart. Remove the heart from 

 the body, cutting its veins and arteries a short distance from it, 

 and dissect it under water. Pin the heart with the ventral side 

 uppermost and thoroughly dissect away the loose tissues from the 

 stumps of the aorta and the pulmonary artery. Identify the liga- 

 mentum Botalli, which joins the two, and cut it. Note at the base 

 of the aorta the beginning of the coronary arteries. Pin the heart 

 with the dorsal side uppermost ; identify and clean the stumps of 

 the precaval, postcaval, and pulmonary veins. 



Slit open the dorsal wall of the right auricle and its appendage, 

 but without injuring the caval veins, and wash out the blood in- 

 side. Note the network of muscles on the inner surface ; also the 

 openings of the caval veins into the auricle. Just posterior to the 

 postcaval opening is that of the small coronary vein, in front of 

 which is a valve. Observe the auricular septum, which separates 

 the two auricles, and the oval depression, the fossa ovalis. In the 

 embryo there is present at this place an opening called the foramen 

 ovale, through which the fetal blood passes directly from the 

 right to the left auricle, instead of going first through the right 

 ventricle and to the lungs. Note the large auriculo- ventricular 

 aperture into the right ventricle. 



Open the right ventricle by slitting the pulmonary artery longi- 

 tudinally and continuing the incision to the hinder end of the 

 ventricle. Note the thinness of the walls of the ventricle, and the 

 muscular ridges on the inner surface. Guarding the auriculo- 

 ventricular aperture is the large tricuspid valve, which consists 

 of a long membranous flap attached at its free end to the wall 

 of the ventricle by long tendinous and muscular cords, the chordae 

 tendinege. The opening of the ventricle into the pulmonary artery 

 is guarded by the three pouched semilunar valves, which permit 

 the flow of blood in only one direction. 



