20 OUTLINES OP EQUINE ANATOMY. 



whicli the cells directly derive the plasma are the capil- 

 laries, extremely minute vessels, uniform in size, forming 

 an intricate network, varying in the figure of its inter- 

 spaces in the different tissues, which in muscle are elon- 

 gated parallelograms, in lungs smaller than the vessels 

 themselves. These vessels connect the terminal extremi- 

 ties of the arteries with the commencement twigs of the 

 veins ; the blood does not take any definite course 

 through them. The wall of the capillary consists of 

 simple membrane with more or less numerous nuclei 

 imbedded in its substance. When the vessels are large 

 they present nuclei elongated both longitudinally and 

 transversely ; the smaller vessels have only longitudinal 

 nuclei, which are widely scattered. The brain and the 

 mucous membrane of the bowels present extremely minute 

 capillaries, those of the skin are very large (as may be 

 well seen in that part modified to form the hoof) . In the 

 spleen, corpus cavernosum penis, and in the chorion, the 

 arteries open directly into veins usually modified to form 

 venous sinuses. 



The arteries carry blood from the heart to the tissues. 

 They vary in size in proportion to their distance from the 

 heart. They may be distinguished from the veins by 

 their elasticity, which enables them to remain open after 

 death, and also to expel the blood which they contained 

 when the heart's action ceased, whence they are found 

 empty. The sum of the areas of the arteries at a distance 

 from the heart is greater than the area of the aorta, and 

 the size of this sum is great in proportion to the distance 

 from the heart, so that the arterial system has fancifully 

 been described as a cone, the apex of which is at the 

 aorta, the base at the capillaries, in which the smallest 

 arteries terminate. Arteries may be given off at right 

 angles or at an acute angle from a large trunk ; as the 

 pressure of fluids in closed vessels is equal in all directions, 

 this arrangement will scarcely influence the flow of blood to 

 particular parts. Some arteries run directly to the organs 

 which they supply, others, especially the spermatic artery 

 in the horse, assume a peculiar convoluted character ; 

 while in some cases several vessels unite to form a more 

 or less intricate plexus termed rete mirabile, examples of 

 which may be seen in the circle of Willis and the circular 

 arteriosus of the foot. Arteries possess three coats. The 



