90 BIOLOGY OF THE LABORATORY MOUSE 



routinely. Hematoxylin and eosin (H & E) stains were used, unless other- 

 wise stated. 



Circulatory System 



The blood vessels. — The walls of the blood vessels are formed of three 

 parts: the innermost part, the interna or intima; the middle part, the media; 

 and the outer part, the adventitia or externa. 



The interna of the arteries consists of the endothelial lining, composed of 

 elongated flat cells with prominent oval nuclei, beneath which is a network 

 of elastic fibers forming the internal elastic membrane. The media is wide 

 and in the large arteries consists of alternating layers of circular smooth 

 muscle fibers and elastic membranes. According to Lowenthal (in Jaffe, 56) , 

 in the aorta six to ten such layers are present, intermingled with fine col- 

 lagenous fibers. In smaller arteries the media contains less elastic and more 

 muscular elements. In the arterioles it consists of only a few individual 

 muscle fibers. The adventitia is composed of loose connective tissue fibers 

 and serves as a transition zone between the arteries and the surrounding 

 tissue. 



The capillaries are formed of elongated endothelial cells usually separated 

 from the surrounding elements by a thin sheath of connective tissue. They 

 connect the terminal arteries with veins. 



In some organs the connection between arteries and veins is accomplished 

 through irregular spaces, the sinusoids. The walls of the sinusoids unlike 

 the capillaries do not have a continuous endothelial cell lining, but are lined 

 by scattered phagocytic and non-phagocytic cells. 



The intima of the veins consists of polygonal-shaped endothelial cells and 

 connective tissue fibers intermingled with fine elastic fibers. The media is 

 formed of smooth muscle fibers and is poorly delimited from the next layer. 

 The media of the wall of pulmonary veins contains cardiac muscle. The 

 adventitia is well developed and contains connective tissue fibers inter- 

 mingled with some longitudinal smooth muscle fibers. The valves of veins 

 are formed of a connective tissue membrane containing a network of elastic 

 fibers. Both surfaces of the valve are covered by endothelium. 



The walls of all of the larger blood vessels are supplied with blood b>- 

 small vessels, the vasa vasorum. In general the walls of veins in relation to 

 the diameter of the lumen are thinner than the walls of the arteries. 



The heart. — The heart consists of four chambers, the left and right atria 

 and ventricles. These chambers are lined by endothelial cells which rest on 

 a very thin layer of connective tissue. These together form the endo- 



