342 THE DIGESTIVE APPARATUS IN MAMMALIA. 



cum scribe what is named the isthmus of the fauces, an aperture constantly 

 closed, in consequence of the great development of the soft palate ; it is only 

 dilated for the passage of the alimentary substances passing into .the pharynx. 

 The isthmus of the fauces is, therefore, not merely an opening ; it is a 

 passage which has for its inferior wall the base of the tongue as far as the 

 epiglottis ; for its upper wall the anterior face of the soft palate ; and for 

 its sides the posterior pillars of the latter. 



STRUCTURE. To give the most simple idea of the structure of the soft 

 palate, it might be said that the mucous membrane of the palato and that of 

 the floor of the nasal cavities are prolonged behind the palatine arch, 

 parallel to one another, and become joined towards the free border of this 

 curtain; and it might be further shown that, in the space between these two 

 mucous membranes, there is a fibrous membrane, muscles, a glandular layer, 

 vessels, and nerves ; besides these, there are no other elements in the organi- 

 sation of the soft palate. They may be studied in the following order : 1, 

 Fibrous membrane j 2, Muscles; 3, Mucous membranes ; 4, Vessels and nerves. 



1. Fibrous membrane (Fig. 148, 5). This membrane, remarkable for its 

 power of resistance, forms a real framework for the soft palate, of which it only 

 occupies the anterior moiety. It is attached in front of the palatine arch, 

 and is prolonged posteriorly by a particular musclo, the palato-pharyngeus. 



2. Muscl-s. Of these muscles, which are all pairs, there are those which 

 constitute a layer situated in the middle of the soft palate itself, and 

 representing the intrinsic muscles ; these are the pharyngo-stapliyleus (palato- 

 pharyngeus) and the palato-staphyleus (circumflexus palati). The others, the 

 peristaphyleus, external and internal (tensors palati, external and internal), 

 are only inserted into the organ by their terminal extremities and therefore 

 act as extrinsic muscles. 



PHARYNGO-STAPHYLEUS. (Fig. 148, 6). 

 (Synonym. Palato-pharyngeus. Percivall.) 



In removing the mucous and glandular layers which cover the anterior 

 face of the soft palate, there is exposed a wide and thin muscular fasciculus 

 succeeding the fibrous layer behind, and occupying the posterior half of the 

 entire organ. The fibres of which this muscle is composed, confounded on 

 the median line with those of the muscle on the opposite side, are directed 

 backwards and outwards, the most posterior following the curve of the free 

 border of the curtain. Arriving near the lateral border, they are re- 

 flected upwards, passing between the pharyngcal mucous membrane and 

 the middle constrictor of the pharynx, with which it appears to be con- 

 founded posteriorly ; but with a little attention it can be followed to the 

 superior border of the thyroid cartilage, into which it is inserted after 

 making a somewhat long track under the mucous membrane of the pharynx. 



This muscle stretches the curtain, and draws its free border from the 

 oesophageal inf undibulum during pharyngeal deglutition. 



PALATO-STAPHYLEUS. 



.Synonyms. Staphyleus Girard. Circumflexus palati Percivatt. The azygosuvulx 

 of Man.) 



A small, elongated, cylindrical, bright-red muscle, in opposition, on the 

 median line, to that of the other side, and extending over the inferior surface 

 of the preceding, from the palatine arch to the free border of the soft palate, 

 which it pulls forward and upward to dilate the isthmus of the fauces. It 



