1236 A MANUAL OF ANATOMY 



fibres to the cavernous plexus, through this plexus to the ciliary ganglion, 

 and thence to the dilator pupillae muscle. 



The vaso-constrictor fibres are derived from a variable number of thoracic 

 nerves, commencing at the second and terminating at the eighth. The origin 

 of the vaso-dilator fibres is not definitely known. The vaso-constrictor fibres 

 ascend to the superior cervical ganglion, in which they terminate. From 

 this ganglion they are continued as sympathetic fibres into the plexus around 

 the external carotid artery, and thence along the branches of that vessel. 



The secretory fibres are destined for the submaxillary gland, and are derived 

 chiefly from the second and third thoracic nerves. They also ascend to the 

 superior cervical ganglion, in which they terminate. They are thence con- 

 tinued as sympathetic fibres into the external carotid plexus, from that into 

 the facial plexus, and thence into the submaxillary ganglion, from which they 

 pass into the submaxillary and sublingual glands. 



The accelerator fibres of the heart are chiefly derived from the second and 

 third thoracic nerves, but some also spring from the first, fourth, and fifth 

 nerves. The spinal fibres pass to the middle and lower cervical ganglia, from 

 each of which they are continued as sympathetic fibres. 



The Mouth. 



The mouth, or buccal cavity, extends from the ritna oris in front, 

 where it opens externally on the face, to the isthmus faucium 

 behind, where it opens into the pharynx. It is divided by the upper 

 and lower dental arches into two compartments, the anterior of 

 which is called the vestibule and the posterior the mouth proper. 



The vestibule is bounded anteriorly and externally by the lips and 

 cheeks, and internally by the dental arches and gums. Superiorly 

 and interiorly it is bounded by the reflection of the mucous mem- 

 brane from the lips and cheeks on to the gums. In the median line, 

 above and below, the mucous membrane forms a small vertical fold, 

 called the frenulum lahii. On the inner surface of each cheek the 

 vestibular mucous membrane presents a small papilla opposite the 

 second upper molar tooth, and on this is the minute opening of 

 Stensen's duct. 



The mouth proper lies within the dental arches. It communicates 

 with the vestibule by the interval between the upper and lower teeth, 

 and also by an opening at either side situated behind the last molar 

 tooth. Anteriorly and externally it is bounded by the dental 

 arches and gums, and posteriorly it opens into the pharynx by 

 means of the isthmus faucium. The roof, which is arched, is formed 

 by the hard palate and the velum pendulum palati, or soft palate. 

 The fioor is formed by the tongue, and the reflection of mucous 

 membrane from the inner surface of the lower dental arch on to 

 its under surface. In the region of the tip of the tongue the 

 lateral reflections of mucous membrane from the two sides of the 

 lower dental arch are continuous across the median line. In the 

 median line the mucous membrane forms a prominent fold, called 

 the frenum lingua, which is connected above with the under 

 surface of the tongue a little distance from the tip. In some children 

 it may extend quite to the tip, impairing the utility of the organ, 

 and giving rise to the condition known as ' tongue-tied.' Close to 

 each side of the frenum there is a small papilla, on which is the 



