MANUAL FOR STABLE SERGEANTS. 



33 



material, the blood receiving waste in the form of carbonic acid gas. 

 An exception to this process is found in the capillaries surrounding 

 the air cells of the lungs, where the blood gives off carbonic acid gas 

 and receives oxygen. The capillaries form a close network in all 

 the tissues of the body, but are invisible to the naked eye. 



Pidmowiry mn-s 



Riijht 



FIG. 16. Section of heart of horse. Specimen hardened in situ and cut nearly at 

 right angles to the ventricular septum. The left ventricle is contracted, but not 

 ad maximum. V. a., Segment of aortic valve. (From Sisson's Anatomy of the 

 Domestic Animals; copyright, W. B. Saunders Co.) 



70. Veins are the vessels that return the blood to the heart. 

 They begin at the capillaries and by uniting form larger veins which 

 finally empty into the atria. Veins differ from arteries in that 

 their walls are thinner and less firm, and by their having valves 

 which prevent the blood flowing backward within them. Veins 



