THE ARTERIES. — THE VEINS. HM 



passive receptacle for tlio blood, and the current must Lave l)cen continued 

 without any or with hardly any fresh impulse.' 



Of air in the heart destrojnng the horse there ai-e some interesting 

 accounts ; and also of rupture of the heart, and anein-ism, or dilatation of 

 the aorta, both thoracic and abdominal, and even farther removed from the 

 heart and in the iliac artery. The symptoms that would certainly indicate 

 the existence of aneurism are yet unknown, except tenderness about the 

 loins and gradual inability to work are considered as such : but it is 

 interesting to know of the existence of these lesions. Ere long the vete- 

 rinary surgeon may possibly be able to guess at them, although he will 

 rarely have more power in averting the consequences of aneurism than the 

 human surgeon possesses with regard to his patient. 



This will be the proper place to describe a little more fully the circula- 

 tion of the blood, and various circumstances connected with that most 

 important process. 



THE ARTERIES, 



The vessels which carry the blood from the heart are called ai'teries 

 (Jceepmg air, — the ancients thought that they contained air). They are 

 composed of three coats : the outer or elastic is that by which they arc 

 enabled to }*ield to the gush of blood, and enlarge their dimensions as it is 

 forced along them, and by which also they contract again as soon as the 

 stream has passed ; the middle coat is a muscular one, by which this con- 

 traction is more powerfully performed, and the blood urged on in its 

 course ; the inner or membranous coat is the mere lining of the tube. 



The arteries divide as they proceed through the frame, and branch out 

 into innumerable minute tubes, termed capillaries (hair-like tubes), and 

 they even become so small as to elude the sight. The slightest puncture 

 cannot be inflicted without wounding some of them. In these little tubes 

 the nourishment of the body and the separation of all the various secre- 

 tions is pei'formed, and in consequence of this the blood is changed. 

 "^Vhen these capillaries unite together, and begin to enlarge, it is found to 

 be no longer arterial or of a florid red colour, but venous, or of a blacker 

 hue ; therefore the principal termination of the arteries is in the veins. 

 The point where the one ends and the other commences cannot be 

 ascertained. It is when the red arterial blood, having discharged its 

 function by depositing the nutritious parts, is changed to venous or black 

 blood. 



THE VEINS. 



These vessels cany back to the heart the blood-s\diich had been conveyed 

 to the different parts by the arteries. They have the same number of 

 coats as arteries, but are thin and comparatively weak. They are more 

 numerous and much larger than the arteries, and consequently the blood, 

 lessened in quantity by the various secretions separated from it, flows more 

 slowly throiigh them. It is forced on partly by the first impulse commu- 

 nicated to it by the heart, also in the extremities and external portions 

 of the frame, by the pressure of the muscles ; and in the cavity of the 

 chest, its motion is assisted or principally caused by the sudden expansion 

 of the ventricles of the heart, after they have closed upon and driven out 

 their contents, and thereby causing a vacuum which the blood rushes on 

 to fill. There are curious valves in various parts of the veins, which pre- 

 vent the blood from tiowinff bacl'Tn-ard to its source. 



