34 Walter Sf<ij,lei/ : 



splints the area whilst the rib accustoms the structures, into 

 which it is tlirust, to the annoyance of the invasion. 



I have prefaced my remarks on the guttural ])0uches of 

 horses by this reference to force operating on tho chest because 

 it sheds some sidelight on the more hidden force effects calling 

 into existence the hernial modifications of the auditory tube of 

 the horse. Between the lateral and median fibro-cartilaginous 

 laminae of the auditory tube the mucous membrane of the 

 tube finds its exit, in sac form, into the retropharynx. Such 

 an escape of the mucous implies an atrophy of the membranous 

 lamina of the auditory tube. The cause of this atrophy cannot 

 at once be seen by dissection. Searching these tissues to find 

 the cause of this obliteration of the membranous lamina we are 

 struck by two remarkable features in and about the throat of 

 the horse. First the narrowness is almost as striking as the 

 great depth of the inter mandibular space : secondly, the stylo- 

 hyoid bone (epihyal) reaches its highest development in the 

 horse. Upon these two facts lai'gely depends the development 

 of the guttural pouches. TTie mucous membrane of the tube 

 has probably been dragged out of the tube by adhesion of tho 

 stylo hyoid bone with the membranous lamina and through it 

 adhesion to the mucosa and stylo-hyoid has occurred. The 

 development of depth of the mandible caused a descent of the 

 larynx, with which organ were carried down the stylo-hyoid 

 bones. The nmcous folds once in the retropharynx were spread 

 to their present confines l)y flexion and extension of the atlo- 

 occipital joint, etc., through adhesion of tho submucosa with 

 the surrounding structures. 



Atmospheric pressure has not produced these nuicous sacs. 

 They are to be found in the foetal foal ; they are delicate sacs 

 even in an old adult. Had they been submitted to pressure 

 during life they should show considerable thickening. The en- 

 trance to these sacs is of sufficient size to proliil)it pressure greater 

 than atmospheric within these sacs. 



The narrow and deep space through which })asses the laryngo- 

 pharA'nx is roofed by the base of the skull : walled by the un- 

 yielding branches of the mandible and the modified digastric 

 muscle filling in the space between the wing of the atlas and 

 the cervical border of tho lower jaw ; tloorod i)V the larvnx 



