398 ARTERIES. 



posterior communicating; behind, by the posterior cerebral, and point of the basilar 

 It is by this anastomosis that the cerebral circulation 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 cinereum, the 

 corpora albicantia, and the pars perforata postica. 



The THYROID Axis 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 cesorjhageal 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 Eectus anticus major. 

 It gives branches to the muscles of the neck, which communicate 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 vertebras, 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 supra-spinous fossa. In this situation it lies close 

 to the bone, and ramifies between it and the Supra-spinatus muscle to which it is> 

 mainly distributed, giving off a communicating branch, which crosses the neck of 

 the scapula, to reach the infra-spinous 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 acromial 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 or' 

 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 passes, 

 and is covered by the Platysma, Sterno-mastoid, Omo-hyoid, and Trapezius 

 muscles. 



The superficial cervical ascends beneath the anterior margin of the Trapezius, 

 distributing branches to it, and to the neighboring muscles and glands in the 

 neck. 



