THE HEAD AND NECK 1263 



is olfactory, the other being derived from Meckel's ganglion. More- 

 over, its epithelial lining is similar to that of the olfactory region 

 of the nasal fossa, inasmuch as it contains olfactory cells, the deep 

 poles of which are continuous with olfactory filaments. In such 

 animals it acts as a supplementary organ of smell. 



Development of Organ ol Jacobsoa. — ^This organ is developed as a diverticu- 

 lum of a portion of the ectoderm of the olfactorj- pit. The diverticulum 

 projects upwards and backwards into a recess situated laterally within the 

 lower and anterior part of the septal nasal cartilage, just above the upper end 

 of Stensen's canal (incisor canal). 



The Pharynx. 



The pharynx is situated behind the posterior nares, isthmus 

 faucimn, and larynx, and it extends from the basilar region of the 

 base of the skull to the level of the lower border of the cricoid 

 cartilage of the larynx, where it becomes continuous with the 

 oesophagus. It is a musculo-aponeurotic tube, about 5 inches long, 

 and it attains its greatest width between the base of the skull and 

 the hyoid bone. BeloW the latter level it narrows, and is flattened 

 from before backwards, so as to assimae the form of a transverse 

 cleft, except during the act of deglutition. 



Relations. — Posteriorly it rests upon the bodies and discs of the 

 cersical vertebrae as low" as the sixth, and the prevertebral muscles 

 covered by the prevertebral fascia. Between it and the last-named 

 fascia is the retropharyngeal space. Which is occupied by connective 

 tissue, but this is so loosely arranged that no obstacle is offered to 

 the movement of the tube, and a post-pharyngeal abscess can 

 readily diffuse itself. Anteriorly it communicates with (i) the nasal 

 fossae through the posterior nares, (2) the Eustachian tubes, (3) the 

 buccal cavity through the isthmus fauciimi, and (4) the larynx. 

 On this aspect its attachments are effected by means of the con- 

 strictor muscles in its w'alls. Laterally it is related to the principal 

 bloodvessels and nerves of the neck, and comes into contact with 

 the styloid muscles and the process from which they arise. Superiorly 

 it is attached to the basilar region of the base of the skulL Inferiorly 

 it is continuous with the oesophagus. 



The w'all of the pharjmx consists of the following four strata, in 

 order from without inwards: (i) the pharjTigeal portion of the 

 bucco-pharyngeal fascia, (2) the muscular coat, (3) the pharyngeal 

 aponeurosis, and (4) the mucous coat. 



Pharyngeal Fascia. — This is an offshoot of the prevertebral 

 lamina of the deep cervacal fascia along the mesial aspect of the 

 carotid sheath. In front it becomes continuous with the fascia 

 covering the buccinator muscle. In association with this fascial 

 stratum there are many veins upon the posterior and lateral Walls 

 of the pharjmx, which constitute the pharvTigeal venous plexus. . 



Muscular Coat. — This is composed, on either side, of the three 

 constrictor muscles, inferior, middle, and superior, the stylo- 

 phar\Tigeus, and the palato-pharyngeus (including the salpingo- 

 pharyngeus). 



