DEVELOPMENT OF THE PHAKYNX AND NECK. 39 



They are supplied by the facial, the nerve of this arch. The 

 muscle bud, from which the whole platysma sheet is developed, is 

 still confined to the area of the hyoid arch at the end of the 

 second month. During the third month the bud spreads out and 

 forms a continuous muscular hood over the head and neck. To 

 this hood or sheet, which is composed of two layers, a deep and 

 superficial, the name of platysma sheet may be given. It is 

 developed in the superficial fascia. 



In man, the platysma sheet has undergone marked retrograde 

 changes in the neck, scalp and external ear, but over the face it 

 has become more highly specialized and differentiated than in any 

 other animal. From this sheet are derived the epicranial 

 aponeurosis, the occipitalis and frontalis. On the face the 

 platysma sheet forms the muscles round the orbit, nose and 

 mouth. The buccinator and levator anguli oris represent parts of 

 the deeper layer of the sheet. The transversus nuchae, fibres 

 occasionally seen in man passing from the middle line of the neck 

 behind, towards the ear and cheek, represent fibres constantly 

 developed in lower primates, and better still in rodents and car- 

 nivores as the sphincter colli and sterno-facialis. 



The muscles supplied by the facial nerve are peculiar in that 

 they are the physical basis into which many mental states are 

 reflected and in which they are realized. Through them mental 

 conditions are manifested. It is found that the differentiation of 

 this sheet into well-marked and separate muscles proceeds pari 

 •passu with the development of the brain. The more highly 

 convoluted the brain of any primate, the more highly specialized 

 are its facial muscles. 



STRUCTUEES DEVELOPED FEOM THE WALLS OF THE 

 PRIMITIVE PHARYNX. 



The Tongue and its Development. — The tongue is developed 

 in the floor of the primitive pharynx between the ventral parts of 

 the 1st, 2nd and 3rd visceral arches (Fig. 31). Two parts are to 

 be recognised in the tongue. The buccal part (Fig. 30) is situated 

 in front of the foramen caecum and the V-shaped groove. It is 

 covered by papillae, concerned in mastication and liable to cancer. 

 The second or pharyngeal part, bounding the buccal part of the 



