2^ OUTLINES OF EQUJNE ANATOMY. 



the circulation. Arteries inosculate or anastomose in some 

 situations freely, but veins rarely approach each other 

 without communication ; they are also more numerouSj 

 much less regular, and larger than arteries. A portion of 

 vein removed from the body will be found more or less 

 coloured by blood which has remained in it after death. 

 The thinness of its walls will allow it to collapse when 

 placed upon a flat surface. Like arteries veins have three 

 coats, which differ from those of the arteries in the paucity 

 of their component elements. The internal or seroid coat, 

 however, requires notice, for, in addition to lining the 

 vessel, it projects into the cavity, double folds constituting 

 valves of a semilunar form. These always have their free 

 margins and concave surfaces directed towards the heart, 

 for they perform an important function in promoting the 

 circulation. According to the size of the vessel from one 

 to three valves may coexist at one particular part. Only 

 two arteries, aorta and pulmonary artery, possess valves, 

 and they look from the heart. Some veins, as those of the 

 foot, portal system, and several components of the ante- 

 rior and posterior venae cavse, have no valves ; in other 

 situations the valves are few in number. When not in use 

 during the forcible rush of blood through the vessels, the 

 valves lie against the sides of the vessel. This is their ordi- 

 nary situation in the jugulars, in which they come into 

 action only when the animal's head is lowered, as in 

 grazing ; under other circumstances the blood passes to the 

 heart in virtue of its own weight. Certain modifications 

 of veins, termed venous simises, will be noticed in due 

 course. 



In addition to blood-vessels we find other vessels which 

 convey a fluid from which blood is prepared. The free 

 spaces of the body, and, according to some authorities, 

 even the serous cavities, are lined by an epithelial mem- 

 brane, continuous with that lining certain vessels termed 

 lymphatics, which unite and reunite until at length they 

 pour their contents into the blood-vessels. These vessels 

 probably run from all parts of the body ; the largest trunks 

 which they form are the tJioracic duct and the great right 

 lymphatic, which open into the large veins at the anterior 

 part of the chest. The smallest lymphatics resemble 

 the capillaries in structure, the largest closely resemble 

 veins, presenting valves, and passing towards the centre of 



