584 THE ARTERIES. 



The postero-medidn ganglionic branches (Fig. 357) are a group of small arteries 

 which arise at the commencement of the posterior cerebral artery ; these, with 

 similar branches from the posterior communicating, pierce the posterior perforated 

 space and supply the internal surfaces of the optic thalami and the walls of the 

 third ventricle. The posterior choroid enters the interior of the brain beneath the 

 posterior border of the corpus callosum and supplies the velum interpositum and 

 the choroid plexus. The postero-lateral ganglionic branches are a group of small 

 arteries which arise from the posterior cerebral artery after it has turned round 

 the crus cerebri ; they supply a considerable portion of the optic thalamus. The 

 terminal branches are distributed as follows : the first to the uncinate gyrus ; the 

 second to the temporo-sphenoidal lobe ; and the third to the cuneus or the occip- 

 ital lobule. 



Circle of Willis. The remarkable anastomosis which exists betAveen the 

 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 

 arteries, branches of the internal carotid, which are connected together by the 

 anterior communicating ; behind, by the two posterior cerebrals, branches of the 

 basilar, which are connected on each side with the internal carotid by the pos- 

 terior communicating arteries (Fig. 354). It is by this anastomosis that the cere- 

 bral 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 

 posterior perforated space. 



The Thyroid Axis (Fig. 347) is a short thick trunk which arises from the fore 

 part of the first portion of the subclavian artery, close to the inner border 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 upward, in a serpentine course, behind the 

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

 ganglion resting upon it), and in front of the vertebral artery, recurrent laryngeal 

 nerve (sometimes behind the nerve), and Longus colli muscle, and is distributed 

 to the posterior surface of the thyroid gland, anastomosing with the superior thy- 

 roid and with the corresponding artery of the opposite side. Its branches are the 



Inferior Laryngeal. (Esophageal. 



Tracheal. Ascending Cervical. 



Muscular. 



The inferior laryngeal branch ascends upon the trachea to the back part of the 

 larynx, in company with the recurrent laryngeal nerve, and supplies the muscles 

 and mucous membrane of this part, anastomosing with the branch from the oppo- 

 site side and with the laryngeal branch from the superior thyroid artery. 



The tracheal branches are distributed upon the trachea, anastomosing below 

 with the bronchial arteries. 



The cesophageal branches are distributed to the oesophagus, and anastomose 

 with the oesophageal branches of the aorta. 



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 

 on the anterior tubercles of the transverse processes of the cervical vertebrae in 

 the interval between the Scalenus anticus and Rectus capitis anticus major. It 

 gives branches to the muscles of the neck, which anastomose with branches of the 

 vertebral, and sends one or two branches into the spinal canal through the inter- 

 vertebral foramina to be distributed to the spinal cord and its membranes, and to 

 the bodies of the vertebrae in the same manner as the lateral spinal branches from 

 the vertebral. It anastomoses with the ascending pharyngeal artery. 



The muscular branches supply the depressors of the hyoid bone, the Longus 

 colli, the Scalenus anticus, and the Inferior constrictor of the pharynx. 



