186 HUMAN PHYSIOLOGY. 



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 semicircular canals and 

 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 auditory meatus by 

 the membrana tympani. 



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

 column, is a cartilaginous rod situated beneath the medullary 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 ver- 

 tebrae, which send processes upward and around the medullary groove, and 

 downward and around the chorda dorsalis, forming in these situations the 

 arches arid bodies of the future vertebrae. 



More externally the outer layer of the mesoblast and epiblast arch down- 

 ward and forward, forming the ventral laminae, in which develop the 

 muscles and bones of the abdominal walls. 



The true cranium'vs, 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 centres of ossification appear, the basi- occipital, the 

 basi-sphenoidal, and the pre-sphenoidal. They ultimately develop into the 

 basilar process of the occipital bone and the body of the sphenoid. 



The entire skeleton'^ at first either membranous or cartilaginous. At the 

 beginning of the second month centres 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. 



Face and Visceral Arches. In the facial and cervical regions the 

 visceral laminae send up three processes, the visceral arches, separated by 

 clefts, the visceral clefts. 



T&z first, or the mandibular arches, unite in the median line to form the 

 lower jaw, and superiorly form the malleus. A process jutting from its 

 base grows forward, unites with the fronto-nasal process growing from 



