THYROID AXIS AND BRANCHES. 475 



branches of the internal carotid and vertebral arteries at the base of the brain, 

 constitutes the circle of Willis. It is formed, in front, by the anterior cerebral 

 and anterior communicating arteries ; on each side, by the trunk of the internal 

 carotid, and the posterior communicating ; behind, by the posterior cerebral, 

 and point of the basilar. It is by this anastomosis that the cerebral circula- 

 tion is equalized, and provision made for effectually carrying it on if one or 

 more of the branches are obliterated. The parts of the brain included within 

 this arterial circle are, the lamina cinerea, the commissure of the optic nerves, 

 the infundibulum, the tuber cinereurn, the corpora albicantia, and the pars per- 

 forata postica. 



The Thyroid Axis (Fig. 274) is a short thick trunk, which arises from the 

 fore part of the first portion of the subclavian artery, close to the inner side 

 of the Scalenus Anticus muscle, and divides, almost immediately after its 

 origin, into three branches, the inferior thyroid, suprascapular, and transversalis 

 colli. 



The Inferior Thyroid Artery passes upwards, in a serpentine course, behind 

 the sheath of the common carotid vessel and sympathetic nerve (the middle 

 cervical ganglion resting upon it), and is distributed to the under surface of the 

 thyroid gland, anastomosing with the superior thyroid, and with the correspond- 

 ing artery of the opposite side. Its branches are the 



Laryngeal. (Esophageal. 



Tracheal. Ascending cervical. 



The laryngeal branch ascends upon the trachea to the back part of the larynx, 

 and supplies the muscles and the mucous membrane of this part. 



The tracheal branches are distributed upon the trachea, anastomosing below 

 with the bronchial arteries. 



The cesophageal branches are distributed to the oesophagus. 



The ascending cervical is a small branch which arises from the inferior 

 thyroid, just where that vessel is passing behind the common carotid artery, 

 and runs up the neck in the interval between the Scalenus Anticus and Rectus 

 Anticus Major. It gives branches to the muscles of the neck, which commu- 

 nicate with those sent out from the vertebral, and sends one or two through 

 the intervertebral foramina, along the cervical nerves, to supply the bodies of 

 the vertebra, the spinal cord, and its membranes. 



The Suprascapular Artery, smaller than the transversalis colli, passes obliquely 

 from within outwards, across the root of the neck. It at first lies on the lower 

 part of the Scalenus Anticus, being covered by the Sterno-mastoid ; it then 

 crosses the subclavian artery, and runs outwards behind and parallel with the 

 clavicle and Subclavius muscle, and beneath the posterior belly of the Omo- 

 hyoid, to the superior border of the scapula, where it passes over the transverse 

 ligament of the scapula to the supraspinous fossa. In this situation it lies 

 close to the bone, and ramifies between it and the Supraspinatus muscle, to 

 which it is mainly distributed, giving off a communicating branch, which crosses 

 the neck of the scapula, to reach the infraspinous fossa, where it anastomoses 

 with the dorsal branch of the subscapular artery. Besides distributing branches 

 to the Sterno-mastoid, and neighboring muscles, it gives off a supra-acromial 

 branch, which, piercing the Trapezius muscle, supplies the cutaneous surface of 

 the acromion, anastomosing with the acrornial thoracic artery. As the artery 

 passes across the suprascapular notch, a branch descends into the subscapular 

 fossa, ramifies beneath that muscle, and anastomoses with the posterior and 

 subscapular arteries. It also supplies the shoulder-joint. 



The Transversalis Colli passes transversely outwards, across the upper part of 

 the subclavian triangle, to the anterior margin of the Trapezius muscle, beneath 

 which it divides into two branches, the superficial cervical, and the posterior 

 scapular. In its passage across the neck, it crosses in front of the Scaleni 

 muscles and the brachial plexus, between the divisions of which it sometimes 



