The weakest part of the pharynx is
(A) Sinus of morgagni (B) Between thyropharyngeal and cricopharyngeal sphincter (C) Piriform fossa (D) Pharyngeal recess
(B) In the posterior wall of the pharynx, the lower part of the thyropharyngeus is a single sheet of muscle, not overlapped internally by the upper and middle constrictors. This weak part lies below the level of the vocal folds or upper border of the cricoid lamina and is limited inferiorly by the thick cricopharyngeal sphincter. This area is knows as Killian’s dehiscence.

 Pharyngeal diverticula are formed by outpouching of the dehiscence. Such diverticula are normal in the pig. Pharyngeal diverticula are often attributed to neuromuscular incoordination in this region which may be due to the fact that different nerves supply the two parts of the inferior constrictor.
 The propulsive thyropharyngeus is supplied by the pharyngeal plexus, and sphincteric cricopharyngeus, by the recurrent laryngeal nerve. If the cricopharyngeus fails to relax when the thyropharyngeus contracts, the bolus of food is pushed backwards, and tends to produce a diverticulum.