1110 



SPLANCHNOLOGY 



exists between the two parts of the cloaca below the urorectal septum; this duct 

 occasionally persists as a passage between the rectum and urethra. The anal 

 canal is formed by an invagination of the ectoderm behind the urorectal septum. 

 This invagination is termed the proctodeum, and it meets with the entoderm of the 

 hind-gut and forms with it the anal membrane. By the absorption of this membrane 

 the anal canal becomes continuous with the rectum (Fig. 993). A small part of the 

 hind-gut projects backward beyond the anal membrane; it is named the post-anal 

 gut (Fig. 991), and usually becomes obliterated and disappears. 1 



Ureter 



Wolffian duct 

 Miillerian duct 



Bladder 

 Symphysis pubis 



Qlans penis 



Urethra 



Vertebral column 

 Fio. 993. Tail end of human embryo, from eight and a half to nine weeks old. (From model by KeibelJ 



THE MOUTH (CAVUM ORIS; ORAL OR BUCCAL CAVITY). 



The cavity of the mouth is placed at the commencement of the digestive tube 

 (Fig. 994); it is a nearly oval-shaped cavity which consists of two parts: an 

 outer, smaller portion, the vestibule, and an inner, larger part, the mouth cavity 

 proper. 



The Vestibule (vestibulum oris} is a slit-like space, bounded externally by the 

 lips and cheeks; internally by the gums and teeth. It communicates with the 

 surface of the body by the rima or orifice of the mouth. Above and below, it is 

 limited by the reflection of the mucous membrane from the lips and cheeks to 

 the gum covering the upper and lower alveolar arch respectively. It receives the 

 secretion from the parotid salivary glands, and communicates, when the jaws are 

 closed, with the mouth cavity proper by an aperture on either side behind the 

 wisdom teeth, and by narrow clefts between opposing teeth. 



The Mouth Cavity Proper (cavum oris proprium) (Fig. 1014) is bounded laterally 

 and in front by the alveolar arches with their contained teeth; behind, it communi- 

 cates with the pharynx by a constricted aperture termed the isthmus faucium. 

 It is roofed in by the hard and soft palates, while the greater part of the floor is 

 formed by the tongue, the remainder by the reflection of the mucous membrane 

 from the sides and under surface of the tongue to the gum lining the inner aspect 

 of the mandible. It receives the secretion from the submaxillary and sublingual 

 salivary glands. 



Structure. The mucous membrane lining the mouth is continuous with the integument at 

 the free margin of the lips, and with the mucous lining of the pharynx behind; it is of a rose- 

 pink tinge during life, and very thick where it overlies the hard parts bounding the cavity. It 

 is covered by stratified squamous epithelium. 



'Consult, in this connection, the following article: "A Contribution to the Morphology of the Human Urino- 

 genital Tract," by D. Berry Hart, M.D., F.R.C.P.E., Journal of Anatomy and Physiology, April, 1901, vol. xxxv. 



