220 HEAD AND NECK 



certain minute branches are given off. The upper ganglion is called the 

 ganglion superius ; the lower one is termed the ganglion pet rosnin. 



The superior ganglion is a small ganglionic swelling, which involves 

 only a portion of the fibres of the nerve trunk. It is placed in the 

 upper part of the bony groove in which the nerve lies as it proceeds through 

 the jugular foramen. No branches arise from it; 



The petrous ganglion is a larger swelling, which involves the entire 

 nerve-trunk, and lies at the opening of the jugular foramen, between the 

 vagus nerve and the inferior petrosal sinus (which intervenes between it 

 and the anterior border of the foramen). Its length is not more than four or 

 five millimetres. Three branches of communication enter or proceed from 

 it and connect it with (i) the superior cervical sympathetic ganglion ; (2) 

 the auricular branch of the vagus ; and (3) the jugular ganglion of the 

 vagus. 



In addition to the twigs mentioned, the tympanic nerve takes origin 

 from the petrous ganglion. 



Nervus Tympanicus. The ultimate destination of the tympanic nerve 

 may be regarded as the otic ganglion, but it takes a very circuitous route 

 to gain that structure, and it gives off branches on the way. It enters a small 

 foramen on the ridge which separates the jugular fossa from the carotid 

 foramen on the lower surface of the petrous bone, and it is conducted by 

 a narrow canal to the tympanic cavity. It crosses the medial wall of that 

 chamber, grooving the promontory. Having gained the anterior part of the 

 tympanum, it enters the bone a second time, and runs in a minute canal, 

 which tunnels the petrous bone below the upper end of the channel which 

 lodges the tensor tympani muscle. In that part of its course the tympanic 

 nerve is joined by a branch from the ganglion geniculi of the facial nerve, 

 and, after the junction is effected, it is termed the lesser superficial petrosal 

 nerve. 



The canal in which the lesser superficial petrosal nerve is lodged opens, 

 by a small aperture, into the cranial cavity, upon the anterior surface of the 

 petrous bone, immediately lateral to the hiatus canalis facialis. Through 

 the aperture the nerve enters the cranial cavity, and it almost immediately 

 leaves it by passing downwards in the interval between the great wing of 

 the sphenoid and the petrous part of the temporal bone, or through the 

 canaliculus innominatus, or through the foramen ovale. Outside the skull 

 it joins the otic ganglion. 



In the tympanic cavity the tympanic nerve gives branches of supply 

 (i) to the mucous membrane of the tympanum ; (2) to the lining 

 membrane of the mastoid cells ; and (3) to the mucous membrane of the 

 auditory tube (Eustachian). It is connected with the sympathetic plexus 

 on the internal carotid artery by the superior and inferior carotico-tympanic 

 branches which pierce the substance of the petrous part of the temporal 

 bone and form with the tympanic nerve the tympanic plexus. 



Nervus Vagus. The vagus nerve passes through the 

 middle compartment of the jugular foramen in company with 

 the accessory nerve both being included within the same 

 sheath of dura mater. In the neck, it pursues a vertical 

 course, lying, at first, between the internal jugular vein and 

 the internal carotid artery, and afterwards between the same 

 vein and the common carotid artery, enclosed within the 

 sheath which envelops the vessels, but on a plane posterior 



