582 Anatomy^ Physiology, and Pathology of the 



proportion to the nearness of the organ they are going to, and in 

 every part, as before stated, they end in tubes so minute as to be 

 named capillaries. As the blood which traverses the arteries is 

 destined to supply the system with the materials necessary for the 

 support of life and development, so do we find that they usually 

 take the most direct course to each particular organ. They also 

 run in parts which are most protected from injury, such as the 

 inner side of limbs and bend of joints. Occasionally also they gain 

 security by passing through canals formed in bones — a fact which 

 can be demonstrated in the skull and the feet of several of our 

 domesticated animals. In number the arteries are considerably 

 less than veins, it being necessary that due provision for the return 

 of blood should be made to balance the circulation, as from the 

 situation of veins the current through them is frequently obstructed. 

 The section of an artery (see fig. 5) shows that its inner coat, 

 which is an epithelial membrane, is, unlike that of a vein, perfectly 

 smooth. Besides this coat, these vessels have four others — a serous, 

 a muscular, an elastic, and a cellular. These tunics are not of 

 equal thickness throughout the course of the artery ; and espe- 

 cially do we find that the elastic is increased in substance the nearer 

 the vessel approaches the heart ; but, on the contrary, that the 

 muscular is most developed at a distance from that organ. The 

 elastic coat gives strength to the vessel, and yields to the distend- 

 ing force of the blood at each stroke of the heart ; but as soon as 

 the volume of the blood has passed, it returns by its inherent pro- 

 perty to its former condition. The expansion and recoil of the 

 elastic coat converts these jets of blood into a continuous stream; 

 but this stream is still augmented in volume at each contraction of 

 the heart. Hence we observe that, when an artery is divided, the 

 flow of blood from its cut end is alternately increased both in quan- 

 tity and force, synchronously with the heart's action. Under these 

 circumstances we likewise perceive that the vessel rises in its bed 

 with a peculiar vermiform action, which proves that the elastic 

 coat is not sim.ply distended, but also elongated by the passing 

 current. The elongation is always greater than the expansion, 

 and the two actions combined produce the arterial pulse. Thus 

 we feel the pulse of the artery when its calibre and length are in- 

 creasing, and that of the heart when its ventricular cavities are con- 

 tracting. In the language of the anatomist, the heart is in a state 

 of systole, and the artery oi diastole, during their respective pulsa- 

 tions. It is not to be inferred from the foregoing remarks that the 

 elastic coat exerts any propulsive power on the passage of the blood, 

 as this can only be effected by lessening the area of the arterial tube, 

 and must consequently depend on its muscular or contractile coat. 

 It is right that I should state that the muscularity of arteries, 

 although strongly insisted upon by the immortal John Hunter, 

 has since his time not been generally admitted. I could adduce 



