852 HUMAN ANATOMY. 



the dura mater, lined by an endothelium similar to and continuous with that of the 

 extracranial veins, but lack any extensive development of elastic fibres in their 

 walls, which are formed by the dura. They possess no valves, although in certain 

 of them, as in the superior longitudinal and cavernous sinuses, the lumen is 

 traversed by irregular trabeculae of fibrous tissue. These are especially well 

 developed and almost tendinous in character in the superior longitudinal sinus, 

 while in the cavernous sinus they are softer, and from them and from the walls of the 

 sinus fringe-like prolongations, .5-2 mm. in length, project freely into the lumen. 

 Connected with certain of these sinuses and developed from certain of the smaller 

 veins which open into them are so-called blood-lakes (lacunae) — cavities or plexuses 

 in the dura mater, lined with endothelium, and connecting either directly or by means 

 of a short canal with an adjacent sinus. They are usually situated more or less sym- 

 metrically with reference to the sinus with which they are connected, and some are 

 very constant in occurrence. Thus, a certain number usually occur on either side 

 of the superior longitudinal sinus (page 1199), others in the tentorium cerebelli con- 

 necting with the lateral sinus, others in the middle fossa of the skull along the course 

 of the meningeal veins, and others in the vicinity of the straight sinus. They occasion- 

 ally reach a considerable size, bulging outward the dura which encloses them and 

 excavating by absorption irregular depressions upon the inner surface of the skull. 

 Occasionally this absorption of the cranial bones proceeds so far that bulging of the 

 outer table of the skull over a lake takes place, and, in the case of those occurring 

 along the course of the superior longitudinal sinus, Pacchionian bodies developed from 

 the subjacent arachnoid tissue may invade them, pushing before them the attenuated 

 floors of the lakes. 



Classification of the Veins. — Theoretically a description of the veins should 

 start with the peripheral vessels and proceed towards the great trunks, following the 

 course of the blood. Such a method would prove, however, somewhat confusing, 

 largely on account of the numerous anastomoses that occur ; it is preferable, therefore, 

 to base a classification primarily upon the great trunks and to consider their afferents 

 topographically, according to the areas which they drain. 



From the embryological stand-point, there are primarily four great systems of 

 veins : ( i ) the cardinal system, represented by the vena cava superior and its tributa- 

 ries ; (2) the inferior caval system; (3) X.\\& portal system ; and (4) t\\Q puhnonary 

 system. Owing to subsequent changes, it is necessary to recognize in the cardinal 

 system three sub-systems : (i) that of the cardiac veins ; (2) that of the superior 

 vena cava and its tributaries, except (3) the azygos veins. In all, then, six great 

 systems of veins may be recognized in the adult. They are as follows : 



1. The pulmonary system. 



2. The cardiac system. "^ 



3. The superior caval system. >- The cardinal system. 



4. The azygos system. ) 



5. The inferior caval system. 



6. The portal system. 



In the descriptions which follow the veins are considered on the basis of this 

 classification. 



THE PULMONARY SYSTEM. 



The Pulmonary Veins. 



The pulrrionary veins (venae pulmonales) (Figs. 749, 750) are four in number, 

 two passing from the hilum of each lung to the posterior surface of the left auricle of 

 the heart. Each vein is formed at the hilum of its lung by the union of a number 

 of smaller vessels which take origin ultimately from the capillary net-work formed by 

 the branches of the pulmonary artery and to a certain extent from that formed by the 

 bronchial arteries. The arrangement of the afferent branches in the substance of the 

 lungs is described in connection with the anatomy of these organs (page 1854), and it 

 will be sufificient to note here that they correspond in number to the branches of the 

 pulmonary artery and of the bronchi, and pursue a course more or less independent 

 of these, which lie side by side. Converging and uniting as they pass towards the 

 hilum, the branches from the superior lobe of each lung unite to form the superior 



