THE HEAD AND NECK 1239 



the descending palatine artery, the branches of which extend 

 forwards and inwards from either lateral angle posteriorly. The 

 mucous membrane of the hard palate is provided with racemose 

 palatal glands of a serous character, which are arranged in two 

 symmetrical groups laterally disposed. 



The lymphatic vessels of the mucous membrane of the hard palate 

 pass to (i) the deep facial, or internal maxillary, glands, and (2) the 

 superior deep cervical glands. 



Nerves. — The great or anterior palatine nerve descends in the 

 posterior palatine canal, from which it emerges through the 

 posterior palatine foramen. Thereafter it divides into branches 

 which pass forwards in grooves on the hard palate, and supply the 

 mucous membrane, glands, and inner aspect of the upper gum. 

 The naso-palatine nerve is distributed to the mucous membrane 

 behind the incisor teeth, where it communicates with twigs of the 

 great or anterior palatine nerve. To reach the hard palate the 

 nerve of the right side passes through ih.e posterior foramen of Scarpa, 

 whilst the left nerve passes through the anterior foramen. The 

 mucous membrane of the cheek is supplied by the long buccal 

 nerve, which is a branch of the inferior maxillary division of the 

 fifth cranial nerve. The mucous membrane of the floor of the mouth 

 derives its nerves from the submaxillary ganglion. 



Arteries. — These are the right and left descending palatine 

 branches of the third part of the internal maxillary. Each descend- 

 ing palatine artery takes the same course as, and has a similar 

 distribution to, the great or anterior palatine nerve. At the 

 anterior palatine fossa it terminates in a small branch which ascends 

 through Stensen's canal, and anastomoses at the upper end of 

 that canal with the naso-palatine branch of the spheno-palatine 

 artery. 



Development of the Mouth, Lips, and Gums. 



The rudiment of the mouth appears as an invagination of the ectoderm 

 of the ventral aspect of the embryo between the fore-brain (which is folded 

 downwards) and the pericardial region. This invagination gives rise to a 

 depression, called the oral fossa or stomodseum. As the stomodaeum deepens, 

 the invaginated ectoderm approaches the phar^'ngeal end of the fore-gut, 

 with which it unites. There is thus formed a bilaminar stratum, called the 

 bucco-pharyngeal membrane, one layer of which is formed by the stomodcsal 

 ectoderm, whilst the other layer consists of the er.toderm of the fore-gut 

 (pharynx). This membrane forms a septum between the pharyngeal part of 

 the fore-gut and the stomodaeum. In the course of the fourth week this 

 septum ruptures and disappears, and thereafter the pharynx communicates 

 freely with the stomodaeum. 



The formation of the stomodaeum and bucco-pharyngeal membrane bears 

 a close resemblance to the formation of the anal fossa or proctodaeum and 

 cloacal membrane, at the caudal end of the hind-gut. 



The primitive oral cavity, or stomodaeum, includes the cephalic part of the 

 pharynx, and from the stomodaeum the permanent oral cavity or mouth 

 and the naisal cavities are formed. 



The boundaries of the stomodaeum are as follows: Dorsally, the bucco- 

 pharyngeal membrane; superiorly and ventrally, the fronto-nasal process; 

 tnferiorly and behind, the two mandibular arches; and, laterally, the corre- 

 sponding maxillary process. 



