GENERAL ANATOMY. 457 



through the whole thickness of the outer coat ; but in the other veins mentioned, 

 a layer of connective and elastic tissues is found external to the muscular fibres. 

 All the large veins which open into the heart are covered for a short distance by 

 a Liver of muscular tissue continued on to them from the heart. 



Muscular tissue is wanting in the veins : 1. Of the maternal part of the placenta. 

 2. In most of the cerebral veins and sinuses of the dura mater. 3. In the veins 

 of the retina. 4. In the veins of the cancellous tissue of bones. 5. In the venous 

 spaces of the corpora cavernosa. The veins of the above-mentioned parts have 

 an internal epithelial lining, supported on one or*more layers of areolar tissue. 



Most veins are provided with valves, which serve to prevent the reflux of the 

 blood. They are formed by a reduplication of the middle and inner coats, and 

 consist of connective tissue and elastic fibres, covered on both surfaces by epithe- 

 lium ; their form is semilunar. They are attached, by their convex edge, to the 

 wall of the vein ; the concave margin is free, directed in the course of the venous 

 current, and lies in close apposition with the wall of the vein, as long as the current 

 of blood takes its natural course ; if, however, any regurgitation takes place, the 

 valves become distended, their opposed edges are brought into contact, and the 

 current is intercepted. Most commonly two such valves are found, placed opposite 

 one another, more especially in the smaller veins, or in the larger trunks at the 

 point where they are joined by small branches ; occasionally there are three, and 

 sometimes only one. The wall of the vein, immediately above the point of attach- 

 ment of each segment of the valve, is expanded into a pouch or sinus, which 

 givos to the vessel, when injected or distended with blood, a knotted appearance. 

 The valves are very numerous in the veins of the extremities, especially the lower 

 ones, these vessels having to conduct the blood against the force of gravity. 

 They are absent in the very small veins, also in the venae cavaa, the hepatic vein, 

 portal vein and its branches, the renal, uterine, and ovarian veins. A few valves 

 are found in the spermatic veins, and one also at their point of junction with the 

 renal vein and inferior vena cava in both sexes. The cerebral and spinal veins, the 

 veins of the cancellated tissue of bone, the pulmonary veins, and the umbilical 

 vein and its branches, are also destitute of valves. They are occasionally found, 

 few in number, in the azygos and intercostal veins. 



The veins are supplied with nutrient vessels, vasa vasorum, like the arteries; 

 but nerves are not generally found distributed upon them. The only vessels upon 

 which they have at present been traced, are the sinuses of the dura mater ; on the 

 spinal veins; on the venae cava3; on the common jugular, iliac, and crural veins; 

 and on the hepatic veins. (Kolliker.) 



The veins may be arranged into three groups : 1. Those of the head and neck, 

 apper extremity, and thorax, which terminate in the superior vena cava. 2. Those 

 of the lower extremity, pelvis, and abdomen, which terminate in the inferior vena 

 cava. 3. The cardiac veins, which open directly into the right auricle of the heart. 



VEINS OF THE HEAD AND NECK, UPPEE EXTEEMITY, AND 



THOKAX. 



VEINS OF THE HEAD AND NECK. 



The veins of the head and neck may be subdivided into three groups. 1. The 

 veins of the exterior of the head. 2. The veins of the neck. 3. The veins of 

 the diploe and interior of the cranium. 



1. VEINS OF THE EXTERIOR OF THE HEAD. 

 The Veins of the Exterior of the Head are the 



Facial. Temporo-m axillary. 



Temporal. Posterior auricular. 



Internal Maxillary. Occipital. 



