272 



HUMAN PHYSIOLOGY. 



is a product of the epiblast, as is also the cornea. The iris appears 

 as a circular membrane without a central aperture, about the seventh 

 week ; the eyelids are formed between the second and third months. 



The internal ear is developed from the auditory vesicle, budding 

 from the third cerebral vesicle ; the membranous vestibule appears 

 first, and from it diverticula are given off, which become the semi- 

 circular canals and the cochlea. 



The cavity of the tympanum, the Eustachian tube, and the external 

 auditory canal are the remains of the first branchial cleft, the cavity 

 of this cleft being subdivided into the tympanum and external audi- 

 tory meatus by the membrana tympani. 



The Skeleton. The chorda dorsalis, the primitive part of the 

 vertebral column, is a cartilaginous rod situated beneath the medul- 

 lary groove. It is a temporary structure, and disappears as the true 

 bony vertebrae develop. On either side are the quadrate masses of 

 the mesoblast, the primitive vertebrae, which send processes upward 

 and around the medullary groove, and downward and around the 

 chorda dorsalis, forming in these situations the arches and bodies 

 of the future vertebrae. 



More externally the outer layers of the mesoblast and epiblasl 

 arch downward and forward, forming the ventral laminae, in which 

 develop the muscles and bones of the abdominal walls. 



The true cranium is an anterior development of the vertebral 

 column, and consists of the occipital, parietal, and frontal segments, 

 which correspond to the three cerebral vesicles. The base of the 

 cranium consists, at this period, of a cartilaginous rod on either side 

 of the anterior extremity of the chorda dorsalis, in which three 

 centers of ossification. appear, the basi-occipital, the basisphenoid, and 

 the presphenoid. They ultimately develop into the basilar process of 

 the occipital bone and the body of the sphenoid. 



The entire skeleton is at first either membranous or cartilaginous. 

 At the beginning of the second month centers of ossification appear 

 in the jaws and clavicle; as development advances the ossific points in 

 all the future bones extend, until ossification is completed. 



The limbs develop, from four little buds projecting from the sides 

 of the embryo, which, as they increase in length, separate into the 

 thigh, leg, and foot, and the arm, forearm, and hand ; the extremities 

 of the limbs undergo subdivision, to form the fingers and toes. 



