37-i ' THE DIGESTIVE APPARATUS JiV MAMMALIA. 



internal pterygoid and stylo-hyoid muscles, the glosso-pharyngeal, great 

 hypoglossal, and superior laryngeal nerves, and the glosso-facial artery. 



STRUCTURE. The walls of the pharynx are composed of a mucous mem- 

 brane, external to which is a muscular layer. 



1. Mucous membrane. This membrane is covered, externally, by a thin 

 layer of yellow elastic fibres, and is much more delicate and less protected 

 by its epidermis than the buccal mucous membrane, of which it is a con- 

 tinuation ; it also communicates with that of the oesophagus, the larynx, the 

 nasal fossae, and the Eustachian tubes. 



Its epithelium is stratified throughout ; but it is thin and ciliated in the 

 upper part, thicker and tesselated on the inferior moiety, which more 

 particularly belongs to the digestive apparatus. 



Everywhere there are racemose glands, though they are most numerous 

 towards the roof of the pharynx. There are also some follicular glands 

 beneath the mucous membrane, in the neighbourhood of the guttural opening, 

 the nasal cavities, and the Eustachian tubes. 



2. Muscular layer. This is composed of seven pairs of muscles, indicated 

 in the following enumeration : the palato-pharyngeus, pterygo-pharyngeus, 

 hyo-pharyngeus, thyro-pharyngeus, crico-pharyngeus, aryteno-pharyngeus, stylo- 

 pharyngeits. 



PALATO-PHARYNGEUS (Pharyngo-staphylinus). This muscle, which has 

 already been described as belonging to the soft palate, is prolonged back- 

 wards on the lateral wall of the pharynx, where its fibres are mixed with 

 those of the pterygo-pharyngeus, and go to be attached to the superior 

 border of the thyroid cartilage by passing beneath the hyo-pharyngeal 

 and thyro-pharyngeal muscles. It therefore also belongs to the pharynx. 



PTERYGO-PHARYNGEUS, OR SUPERIOR CONSTRICTOR (tliepalato-pharyngeusof 

 Percivall). This muscle is thin, wide, flat, and triangular. It originates 

 from the pterygoid process, whence its fibres diverge, some posteriorly, 

 others inwardly. The former mix with those of the palato-pharyngeus, 

 and comport themselves like that muscle; and the latter are united, on 

 the median line, with the analogous fibres of the opposite muscle, form- 

 ing a kind of zone around the origin of the Eustachian tube. This 

 muscle is covered, externally, by a layer of yellow elastic tissue, which is 

 attached with it to the pterygoid bone ; afterwards it is fixed to the superior 

 border of the great branch of the os hyoides, and is even prolonged on the 

 external surface of the muscle it covers to the thyroid cartilage. 



The elasticity of this fibrous covering plays a certain part in the move- 

 ments of the hyo-laryngeal apparatus, in acting as a passive antagonist of its 



This muscle is, and can only be, a perfect constrictor of the pharynx, 

 as it diminishes the diameter of that cavity in every direction : the longi- 

 tudinal diameter, by means of its posterior fibres, which draw the thyroid 

 cartilage forward ; and its transverse diameter, by the circle thrown around 

 the orifice of the Eustachian tubes (Figs. 149 ; 176, 8). 



HYO-PHARYNGEUS, OR FIRST MIDDLE CONSTRICTOR ; THYRO-PHARYNGEUS, 

 OR SECOND MIDDLE CONSTRICTOR ; AND CRICO-PHARYNGEUS, OR INFERIOR 

 CONSTRICTOR. The two first of these muscles only form one in Man, the 

 middle constrictor of the pharynx. They are three muscular bands which 

 terminate above the pharynx, on a median fibrous fold sometimes wide 

 enough to look like an aponeurosis. The first band arises from the cornu of 

 the os hyoides ; the second, from the external surface of the thyroid carti- 

 lage ; the third, from the superficial face of the cricoid cartilage. 



