968 SPECIAL PHYSIOLOGY. 



the facial maxillary lobe. Sometimes these parts are arrested in de- 

 velopment, and fail to unite properly, giving rise to the conditions of 

 harelip and cleft palate. Certain other congenital defects, connected 

 chiefly with the apertures of the body, as well as with the back of the 

 head and spine, are explained by similar arrests of normal adhesive 

 processes of development. 



From the middle- part of the first branchial arch, to which we now 

 return, the incus of the tympanum is developed. From it, also, a re- 

 markable cartilaginous process, named MeckeVs process, or MeckeTs 

 cartilage, arises, which gives origin to the malleus* and also extends 

 forwards from that bone to the rudimentary lower jaw, which is de- 

 veloped independently upon it, after the manner of the opercular bones 

 of the cranium, which rest upon the basal bones. Afterwards, Meckel's 

 process wastes, except a part, which forms the processus gracilis 

 of the malleus. From the second branchial arch are developed the 

 stapes, from a minute cartilage, and also the stapedius muscle, with 

 its bony canal ; these belong to the tympanic cavity. In the neck, 

 the second arch forms the styloid process, the stylohyoid ligament, and 

 the little cornu of the hyoid bone, which early unites with the tongue. 

 The cartilage of the third branchial arch gives origin to the great 

 cornu and body of the hyoid bone ; but the arytenoid cartilage and 

 the epiglottis are developed from the first arch. The fourth branchial 

 arch soon coalesces with the side of the neck. All these changes 

 occur very early in the pulmonated Vertebrata. 



The Alimentary Canal. 



The digestive canal is at first merely the interior of the body-cavity, 

 which is formed by the folding dow r nwards and inwards of the lateral 

 ventral plates, and which, originally, communicates widely with the 

 yolk-sac, by the open vitelline duct. The walls of this common body- 

 cavity are derived principally from the middle germinal layer, but they 

 are lined by the inner or intestinal germinal layer. It is a short, 

 straight chamber, closed at both the cephalic and the caudal end of 

 the embryo. Its innermost part soon separates from the sides of the 

 embryo, and forms a tube, in which an abrupt bend occurs opposite 

 the umbilical opening, and for a time projects through it, being there 

 connected with the vitelline duct. In the Bird, this duct continues 

 open, even after the chick is hatched ; but in the Mammalian embryo, 

 it soon becomes closed, and, attached to the primary bend of the ali- 

 mentary canal, forms the 'slender pedicle of the yolk-sac or umbilical 

 vesicle. The part of the canal, or tube above the bend, forms the 

 pharynx, oesophagus, stomach, and a portion of the small intestine ; 

 the part below the bend, the remainder of the small intestine, and the 

 large intestine : the distinction between these is soon indicated by the 

 appearance of a caecum, a little lower down than the bend. The 

 closed upper end of this alimentary tube extends to the base of the 

 cranium, corresponding with the pharynx, the oesophagus being con- 

 tinuous with it below. The lower closed end corresponds with the 

 lower portion of the rectum. At a certain time, there is neither an 



