82 DISSECTION OF THE NECK. 



cross the upper part of the arterial trunk ; and opposite the thyroid body 

 another small vein (middle (thyroid) is directed back over the vessel. 

 Near the clavicle the anterior jugular vein passes out under the sterno- 

 mastoid : it is superficial to the artery, and separated from it by the sterno- 

 hyoid and thyroid muscles. 



Arteries. Offsets of the upper thyroid artery descend over the top of 

 the sheath ; and the inferior thyroid crosses under it below the level of the 

 cricoid cartilage. 



Nerves. The descendens noni lies in front of the sheath, crossing from 

 the outer to the inner side, and is joined there by the cervical nerves. The 

 pueumogastric lies within the sheath, behind and between the artery and 

 the vein. The sympathetic cord and branches rest on the spine behind 

 the sheath. All the nerves above mentioned have a longitudinal direction; 

 but the inferior laryngeal or recurrent crosses obliquely inwards behind 

 the sheath towards the lower end of the artery. 



Branches of carotid. As a rule, the common carotid artery does not 

 furnish any collateral branch, though it is very common for the superior 

 thyroid to spring from its upper.end. At the terminal bifurcation into the 

 two carotids the artery is slightly bulged. 



The INTERNAL JUGULAR VEIN extends upwards to the base of the skull, 

 but only the part of it that accompanies the common carotid is now seen. 

 Placed behind or external to its artery, the vein ends below by uniting 

 with the subclavian in the innominate vein. Its proximity to the carotid 

 is not equally close throughout, for at the lower part of the neck the vein 

 inclines backwards, leaving a space between it and the artery, in which 

 the vagus nerve is seen about midway between the two. Sometimes the 

 vein is superficial to the carotid, as on the left side. 



The lower part of the vein is marked by a dilatation or sinus. Near 

 its ending it becomes contracted, and is provided with a pair of valves 

 (Struthers). 



In this part of its course the vein receives the superior and middle thy- 

 roid branches. 



Peculiarities of the carotid. The origin of the artery on the right side 

 may be above or below the point stated. Mention has been made of the 

 difference in the place of bifurcation, and of the fact that the common 

 carotid may not be divided into two (p. 70). Instead of one, there may 

 be two trunks issuing from beneath the hyoid muscles. 



Dissection. The dissector may next trace out completely the trunk of 

 the external carotid (fig. 17), and follow its branches until they disappear 

 beneath different parts. Afterwards he may separate from one another 

 the digastric and stylo-hyoid muscles, which cross the carotid ; and may 

 define their origin and insertion. 



The DIGASTRIC MUSCLE (fig. 15 9 ) consists of two fleshy bellies, united 

 by an intervening tendon, whence its name. The posterior, the larger of 

 the two, arises from the groove beneath the mastoid process ; whilst the 

 anterior belly is fixed on the side of the symphysis of the lower jaw. 

 From these attachments the fibres are directed to the intervening tendon, 

 which is surrounded by fibres of the stylo-hyoideus,' and is united to its 

 fellow, and 'to the body and part of the great cornu of the os hyoides by an 

 aponeurotic expansion. 



The arch formed by the digastric is superficial, except at the outer end, 

 where it is beneath the sterno-mastoid and splenius muscles. The poste- 

 rior belly covers the carotid vessels and the accompanying veins and 



