62 



An artery retains its circular form when empty, while a vein 

 collapses. 



Arteries frequently anastomose with each other to form com- 

 plete circles, especially in the brain and feet. 



A ligature tied round an artery, divides the middle muscular 

 coat first on account of the circular arrangement of its fibres. 



The muscular portion of the middle coat is of the involun- 

 tary class of muscles, and acts by means of nervous force, or 

 stimulus, or by electricity. The internal serous coat is very 

 brittle. It possesses neither blood-vessels nor nerves. 



Arteries are very extensile, that is, they are susceptible of 

 yielding and elongation, and of returning to their original 

 dimensions. 



THE PLANTAR ARTERIES. 



The following account of the Plantar Arteries is derived 

 from Spooner, on "The Foot of the Horse." 



" The Plantar Arteries at their origin above the fetlock form 

 an acute angle from the apex of which gradually receding, 

 they pass over the joint by the side of the sesamoid bones, and 

 somewhat in advance of the tendons, conforming in their pro- 

 gress to the prominence of the fetlock and the depressions of 

 the pasterns. Below the joint, the artery, still pursuing a 

 straight course, is, from the smaller circumference of the pas- 

 terns, compared with the fetlock, again brought very near, and 

 somewhat in advance of the perforatus tendon, and so con- 

 tinues till the termination of the tendon, when the artery itself 

 is hidden from view by the lateral cartilage. It here inclines 

 somewhat forwards, being surrounded by the substance of 

 the cushion, and just within the upper part of the wings of the 

 coffin-bone it enters a groove which conveys it downwards and 

 forwards to the foramen on the posterior concavity of the bone, 

 just under the coffin joint. The artery within the substance of 

 the bone takes a circular direction, and joins its fellow on the 

 other side, from which circumstance it is designated by Pro- 

 fessor Coleman the circulus arteriosus. In the course we have 



