120 THE HEAD AND NECK 



as it indicates the proximity of the vessel. It is drawn down- 

 wards and medially, bringing the middle thyreoid veins into view, 

 and the carotid sheath is opened on its medial side to avoid 

 injuring the internal jugular vein. 



Ligature below the omo-hyoid is performed for aneurism of the 

 distal part of the artery. This operation is more difficult than 

 the preceding one, because the vessel lies more deeply and is 

 placed under cover of the sterno-hyoid and sterno-thyreoid 

 muscles. The incision extends somewhat lower, and after 

 retracting the sterno-mastoid laterally, the surgeon draws the 

 omo-hyoid upwards and laterally, and the sterno-hyoid and 

 sterno-thyreoid downwards and medially. 



In both these operations care must be taken not to include 

 the vagus or the sympathetic trunk in the ligature. If the 

 sympathetic is injured, unilateral sweating of the head and neck, 

 contraction of the pupil, and retrogression of the eyeball will 

 occur on the affected side. 



After ligature of the common carotid the collateral circulation 

 is carried out by (i) the occipital anastomosis ; (2) the thyreoid 

 anastomosis ; (3) the anastomosis which is established by the 

 branches of the external carotid arteries, lingual, external 

 maxillary (facial), etc. across the median plane. 



1. In the occipital anastomosis, the ascending branch of the 

 transverse cervical (p. 126), the profunda cervicis (p. 137), and 

 the ascending cervical (p. 143), which are all derived from the 

 subclavian artery, anastomose with the descending and muscular 

 branches of the occipital artery. This anastomosis takes place 

 on the superficial and deep surfaces of the semispinalis capitis 

 (complexus). 



2. The thyreoid anastomosis occurs between the superior 

 and inferior thyreoid arteries in and around the thyreoid gland. 

 This anastomosis also establishes a connection between the 

 external carotid and the subclavian arteries. 



When the common carotid artery is ligatured, the brain 

 receives a sufficient, though diminished, blood-supply, owing to 

 the free anastomosis established by the arterial circle (of Willis). 

 In infants suffering from hydrocephalus, one common carotid 

 having been ligatured, the other may be tied after a short 

 interval without any deleterious effects on the nutrition of the 

 brain (Stiles), but in adults the ligature of one common carotid 

 alone may produce cerebral softening. 



The External Carotid Artery begins opposite the upper 



