474 PRACTICAL ANATOMY. 



Veins are stronger than arteries proportionate to their thickness. 



The walls of veins arc thinner than those of corresponding arteries. 



The following veins do not accompany arteries : The large veins from bone ; 

 veins from the skull and spinal canal ; the hepatic veins these latter take all 

 blood from the liver to the ascending vena cava. 



The veins in the lower are thicker than those in the upper extremity. 



Some veins have semilunar valves formed by the inner coat. 



The following veins have no valves : The facial, angular, Vesalian, and oph- 

 thalmic ; the intracranial veins and dural sinuses ; the hepatic, renal, spermatic, 

 ovarian, and uterine ; the superior and inferior venae cavae and veins of bone ; 

 the veins of the whole portal system. 



All veins have nutrient or trophic vessels called vasa vasorum. 



Tlie rule governing the relation of veins to arteries is as follows : Above the 

 diaphragm veins are anterior to their arteries ; below the diaphragm, veins are 

 behind their arteries, where they are not on the same plane. Exception to the 

 rule, the renal vessels. 



Name the veins of the head and neck and indicate any points of special or prac- 

 tical importance in connection therewith. 



The occipital vein (Fig. 18) is the principal vein of the posterior region of 

 the scalp. It perforates the trapezius muscle and is tributary to the deep cervi- 

 cal vein. It communicates by an emissary with the lateral sinus by the mastoid 

 foramen. This may explain the rationale of the empiric practice of counterirri- 

 tation in the region of the mastoid. 



The common temporal vein (Fig. 18) communicates with the deep temporal 

 plexus of veins by the deep temporal vein. Infection of the region of the scalp, 

 drained by tributaries of this vein, may reach the cavernous sinus via the deep 

 temporal vein, the pterygoid plexus, and the Vesalian vein. In the parotid gland 

 this vein unites with the internal maxillary to form the temporo-maxillary vein. 



The facial vein (Fig. 18) corresponds in distribution to its companion artery. 

 Near the angle of the jaw it will be seen to communicate with the external jug- 

 ular vein. The vein is tributary to the internal jugular below the jaw; the sub- 

 maxillary gland, the digastric and stylo-hyoid muscles, and the hypoglossal nerve 

 (Fig. 31) intervening between the vein and its artery in this locality. The most 

 important tributary of this vein is the angular, which communicates with the 

 ophthalmic vein. Infection may reach the cavernous sinus via the angular 

 vein. (See p. 151.) 



The external jugular vein (Fig. 18) is the cervical continuation of the tem- 

 poro-maxillary vein. It perforates the deep cervical fascia behind the middle of 

 the clavicle, and opens into the subclavian or into the internal jugular vein. The 

 special feature of this vein is its inability to collapse in the region of the clavicle. 

 The deep cervical fascia prevents collapse. This inability to collapse renders 

 puncture of the external jugular vein liable to the admission of air. 



The internal maxillary vein is the companion of the artery bearing the 

 same name. This vein returns blood from the deep parts of the face, the muscles 

 of maturation, tin- teeth, and the greater part of the dura mater. The veins from 

 these regions form a plexus called the pterygoid. This plexus is between the 

 temporal and external pterygoid muscles, around the latter muscle, and on the 

 inner surface of the internal pterygoid muscle. The internal maxillary vein 

 proper begins at this plexus. It unites with the temporal vein in the parotid 

 -land to form the temporo-maxillary vein. (See p. 151.) 



The pterygoid plexus communicates with the cavernous sinus via the 

 vein of Ves.ilius ; hence infection anywhere in the region of the radicals of the 

 internal maxiHarv vein may reach tin- dural sinuses in the same way. 



The internal jugular vein (Fig. ;, i) he-ins at the ju-ular foramen. It is in 



