x. ] THE CIRCULA TING SYSTEM. 4 1 1 



The heart may show marked signs even externally of the 

 division into a right and left side, as in the Dugong, where 

 its apex is deeply notched. 



The heart may be more elongated than in man, as in Birds ; 

 or shorter and broader, as in Chelonians. 



5. As has been said, two ARTERIES in man leave the heart, 

 one proceeding from the right ventricle, the other from the 

 left. These two channels ultimately end in distinct sets ot 

 capillaries, and thus there are two categories of arterial 

 channels. 



The great artery proceeding from the right side of the 

 heart is termed pulmonary, and conveys the as yet unaerated 

 blood for purification. It soon bifurcates into a right and a 

 left branch, and these branches ramify in the substance of 

 the right and left lungs respectively. 



The arterial trunk proceeding from the left side of the 

 heart is the largest of all, and is called the great aorta. 

 It ultimately ends in capillaries distributed over the whole 

 body, and constitutes the generally diffused (or systemic) 

 arterial system. The aorta arises from the upper and back 

 part of the left ventricle, whence it ascends forwards and to 

 the right, and then curves backwards and towards the left, 

 forming what is called the " arch of the aorta" It curves 

 over the left bronchus, passing obliquely from the sternum to 

 the spine. It then continues on in a nearly straight, vertical 

 line, descending along the front of the vertebral column 

 (rather on its left side) to the fourth lumbar vertebra, where 

 it divides into the two common (or unsubdivided) iliac 

 arteries to be noticed shortly. 



The aorta gives off from the convexity of its arch four 

 great arteries, two going one to each side of the head, the 

 two others to the two arms. Normally, however, the two 

 vessels, going respectively to the right arm and right side of 

 the head, are undivided for a short space, forming a large 

 trunk called the innominate artery. 



The two arteries for the head are each called " common 

 carotid" and each bifurcates in the upper part of the neck 

 into two branches, called respectively the external and the 

 internal carotid. 



The external carotid, besides physiologically important 

 branches to the face and the sides of the head, sends one 

 called the thyroid to the larynx, one called the lingual to the 

 tip of the tongue, and one, the posterior auricular, which as- 

 cends (close to the styloid process, and passing through the 



