536 VEINS. 



Sinuses are venous channels, which, ii\ their structure and mode of distribu- 

 tion, differ altogether from the veins. They are found only in the interior of 

 the skull, and are formed by a separation of the layers of the dura mater ; their 

 outer coat consisting of fibrous tissue, their inner of a serous membrane con- 

 tinuous with the serous membrane of the veins. 



Veins have thinner walls than arteries, the difference in thickness being due 

 to the small amount of elastic and muscular tissues which the veins contain. 

 The superficial veins usually have thicker coats than the deep veins, and the 

 veins of the lower limb are thicker than those of the upper. 



The minute structure of these vessels is described in the Introduction. 



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

 neck, upper extremity, and thorax, which terminate in the superior vena cava. 

 3. Those of the lower limb, pelvis, and abdomen, which terminate in the infe- 

 rior vena cava. 3. The cardiac veins, which open directly into the right auricle 

 of the heart. 



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. 



The Veins of the Exterior of the Head are, the 



Facial. Temporo-maxillary. 



Temporal. Posterior auricular. 



Internal maxillary. Occipital. 



The Facial Vein passes obliquely across the side of the face, extending from 

 the inner angle of the orbit, downwards and outwards, to the anterior margin 

 of the Masseter muscle. It lies to the outer side of the fascial artery, and is 

 not so tortuous as that vessel. It commences in the frontal region, where it is 

 called the frontal vein; at the inner angle of the eye it has received the name 

 of the angular vein; and from this point to its termination is called the facial 

 vein. 



The frontal vein commences on the anterior part of the skull, by a venous 

 plexus, which communicates with the anterior branches of the temporal vein; 

 the veins converge to form a single trunk, which runs downwards near the 

 middle line of the forehead parallel with the vein of the opposite side, and 

 unites with it at the root of the nose, by a transverse trunk, called the nasal 

 arch. Occasionally the frontal veins join to form a single trunk, which bifur- 

 cates at the root of the nose into the two angular veins. At the nasal arch the 

 brunches diverge, and run along the side of the root of the nose. The frontal 

 vein as it descends upon the forehead receives the supraorbital vein ; the dorsal 

 veins of the nose terminate in the nasal arch ; and the angular vein receives 

 the veins of the ala nasi on its inner side, and the superior palpebral veins on 

 its outer side ; it moreover communicates with the ophthalmic vein, which 

 establishes an important anastomosis between this vessel and the cavernous 

 sinus. 



The facial vein commences at the inner angle of the orbit, being a continua- 

 tion of the angular vein. It passes obliquely downwards and outwards, be- 

 neath the great zygomatic muscle, descent Is along the anterior border of the 

 M.isseter, crosses over the body of the lower jaw, with the facial artery, and, 

 passing obliquely outwards and backwards, beneath the Platysma and cervical 

 fascia, unites with a branch of communication from the temporo-maxillary vein, 

 to form a trunk of large size which enters the internal jugular. 



Branches. The facial vein receives, near the angle of the mouth, communi- 

 cating branches from the pterygoid plexus. It is also joined by the inferior 

 palpebral, the superior and interior labial veins, the buccal veins from the 

 cheek, and the masseteric veins. Below the jaw, it receives the submental, the 



