162 HUMAN PHYSIOLOGY. 



above, and forms the upper jaw. When the superior maxillary processes* 

 fail to unite, there results the cleft-palate deformity ; if the integument also 

 fails to unite, there results the hare-lip deformity. The space above the 

 mandibular arch becomes the mouth. 



The second arch develops the incus and stapes bones, the styloid process 

 and ligament, and the lesser cornu of the hyoid bone. The cleft between 

 the first and second arches partially closes up, but there remains an open- 

 ing at the side which becomes the Eustachian tube, tympanic cavity, and 

 external auditory meatus. 



The third arch develops the body and greater cornu of the hyoid 

 bone. 



Alimentary Canal and its Appendages. The alimentary canal is 

 formed by a pinching off of the yelk sac by the visceral plates as they grow 

 downward and forward. It consists of three distinct portions, the fore gut, 

 the hind gut, and the central part, which communicates for some time with 

 the yelk sac. It is at first a straight tube, closed at both extremities, lying 

 just beneath the vertebral column. The canal gradually increases in 

 length, and becomes more or less convoluted ; at its anterior portion two 

 pouches appear, which become the cardiac and pyloric extremities of the 

 stomach. At about the seventh week the inferior extremity of the intestine 

 is brought into communication with the exterior, by an opening, the anus. 

 Anteriorly the mouth and pharynx are formed by an involution of epiblast, 

 which deepens until it communicates with the fore gut. 



The Liver appears as a slight protrusion from the sides of the alimen- 

 tary canal, about the end of the first month ; it grows very rapidly, attains 

 a large size, and almost fills up the abdominal cavity. The hepatic cells 

 are derived from the intestinal epithelium, the vessels and connective 

 tissue from the mesoblast. 



The Pancreas is formed by the hypoblastic membrane. It originates in 

 two small ducts budding from the duodenum, which divide and subdivide, 

 and develop the glandular structure. 



The Lungs are developed from the anterior part of the oesophagus. At 

 first a small bud appears, which, as it lengthens, divides into two branches ; 

 secondary and tertiary processes are given off these, which form the bron- 

 chial tubes and air cells. The lungs originally extended into the abdomi- 

 nal cavity, but became confined to the thorax by the development of the 

 diaphragm. 



The Bladder is formed by a dilatation of that portion of the allantois 

 remaining within the abdominal cavity. It is at first pear-shaped, and 

 communicates with the intestine, but later becomes separated, and opens 



