308 CIRCULATORY SYSTEM. 



The pulmonary valve is sometimes spoken of as the pulmonary 

 semilunar valve or valves. It is composed of three, occasionally 

 two, segments, and is situated at the beginning of the pulmonary 



artery. These segments are at- 

 tached at their bases to the wall of 

 the artery, and on the free edge of 

 each is a projection, the corpus 

 Arantii. When the valve is open 

 the segments lie against the walls 

 of the artery ; when it is shut they 

 are in contact, and thus close the 

 orifice of the pulmonary artery. 



The mitral valve is sometimes 

 described as the bicuspid, because 

 it consists of two cusps. The 

 attachments of the segments, the 



p resenc ? of chord *> r d f other 



great vessels being removed : PA, pul- anatomic points reterred to in 

 monary artery and its semilunar spea ki n cr o f the tricuspid Valve 

 valves: AO, aorta and its valves; r . r 



EAV, tricuspid, and LAV, bicuspid are to be seen in connection with 



valves; MV, segments of mitral valve; the mitral. It closes the auricu- 

 LV, segments of tricuspid valve , . . -, . 



(Huxley), loventncular orifice. 



The aortic valve resembles in 



all essential particulars the pulmonary ; it likewise is sometimes 

 called the semilunar valve, and closes the aortic orifice. 



The ventricular septum is the partition between the right 

 and left ventricles. It is closed at all periods of life. The 

 auricular septum, between the auricles, is closed from the tenth 

 day after birth ; prior to this time and during fetal life it has an 

 opening, the foramen ovale, which serves as a means of communi- 

 cation between the right and the left auricles. 



Structure of the Valves. The valves consist of reduplications 

 of endocardium, between which is fibrous tissue. 



THE ARTERIES. 



Arteries are composed of three coats : (1) An internal, tunica 

 Mima; (2) a middle, tunica media; and (3) an external, teica 

 adventitia. 



The tunica intima consists of a layer of pavement-epithelium 

 (Fig. 160), the cells being polygonal, oval, or fusiform, termed 

 endothelium, and of a network of elastic fibers or a fenestrated 

 membrane (Fig. 161). Between these two layers is a subepithelial 

 layer consisting of connective tissue. 



The tunica media has a special physiologic interest. In the 

 large arteries that is, those larger than the carotids this coat is 

 principally yellow elastic tissue, only about one-fourth being mus- 

 cular tissue. Vessels of this size are therefore characterized by 



