THE COMMON CAROTID ARTERIES. 663 



its track, this artery is crossed, outwardly, by the pneumop^astric and spinal 

 nerves, and the occipital nerve of the threat sympathetic, and is accompanied by 

 the divisions of the inferior branch of the first pair of cervical nerves. 



The two terminal branches of the vessel are the occipito-muscular and cerebro- 

 spinal arteries. 



The collateral branches are three in number, and in the order of their 

 emission are named : 1. The jn-evertebral ortery. 2. The mastoid artery. 3. 

 The atloido-muscuJdr {ramus anastomotic a f<, or rctroyradc) artery. 



Collateral Branches. — 1. Prevertebral Artery (Fig. 381, 9). — The 

 smallest of all the branches emanating from the occipital, this artery is detached 

 at a very acute angle, and immediately divides into several filaments, some 

 mnscular, the others inoiingeal. The majority of the first pass between the 

 occipito-atloid articulation and the rectus capitis anticus minor muscle of the 

 head, and expend themselves either in that muscle, or in the rectus capitis anticus 

 major ; the second, generally two in number, are always very slender, and reach 

 the dura mater by entering, one through the foramen lacerum basis cranii, the 

 other by the condyloid foramen. 



Sometimes the prevertebral artery arises from the common carotid, near the 

 occipital and internal carotid. 



2. Mastoid Artery (Fig. 381, 8). — This vessel arises at an acute angle 

 above the preceding, and goes towards the mastoid foramen by creeping on the 

 external surface of the styloid process of the occipital bone, beneath the obliquus 

 capitis anticus. It enters the parieto-temporal canal by this foramen, to anasto- 

 mose by inosculation with the spheno-spinous branch of the internal maxillary 

 artery. 



In its course it describes a curve downwards, and throws off a large number 

 of collateral branches. Among these are some which originate before the artery 

 enters its bony canal, and which are destined for the muscles of the nape of the 

 neck. Others arise in the interior of this canal, and escape from it by the 

 orifice in the temporal fossa, to expend themselves in the temporal muscle. 

 Some ramuscules reach the dura mater. 



We have seen the mastoid artery arise directly from the common carotid, 

 and furnish a parotideal branch. "We have also found it passing over the 

 surface of the obUquus capitis anticus, and curve suddenly to enter the parieto- 

 temporal canal. 



3. Atloido-muscular (Ramus Anastomoticus) or Retrograde Artery 

 (Fig. 381, 7). — This branch is not constant, and when it does exist it presents 

 a variable volume. It is detached from the occipital, underneath the transverse 

 process of the atlas, by forming with the parent branch a right, or even an 

 obtuse angle ; it is directed backwards, traverses the inferior foramen in the 

 transverse process of the atlas, places itself beneath the atlo-axoid muscle, and in 

 a fiexuous manner advances to meet the vertebral artery, which it directly joins, 

 after giving off some branches to the great oblique and neighbouring muscle. 

 This anastomosis is the means of establishing a collateral communication between 

 the vertebral artery and the divisions furnished by the common carotid ; so that 

 these two arteries can mutually assist or supplant each other.^ 



Terminal Branches. — 1. Occipito-muscular Artery (Fig. 381, 10). — 

 Covered at its origin by the obhquus capitis inferior, the occipito-muscular artery 



' In a Mule we have found a large anaatomosis between the retrograde and mastoid 

 arteries, beneath the ala of the atlaa. 



