CHAP. XLII.] OF THE NERVOUS SYSTEM. 599 



meet, while the intermaxillary bones are united in the centre, and 

 form a prominent tongue of bone, on either side of which is a 

 deep fissure between the intermaxillary and corresponding maxillary 

 bones of each side thus is produced the deformity of double hare- 

 lip. The cleft of the palate in these cases usually remains open, 

 and in this way the malformation is increased. The fissure of the 

 lip seems to arise from the alteration of the deeper parts ; for as 

 such a fissure exists at no period of embryonic life in the soft parts, 

 it cannot, like the bony fissure above described, be dependent upon 

 an arrest of development. 



The first visceral arch gives rise to the superior maxillary appa- 

 ratus, consisting of the intermaxillary bones, the vomer, the maxillary 

 and palate bones, and the pterygoid plates of the sphenoid, the lower 

 jaw, and the malleus and incus. 



The second visceral arch gives origin to the hyoid bone, the styloid 

 process, and its ligaments, and the stapes of the ear. In animals a 

 great part of this hyoid apparatus becomes ossified. 



From the third visceral arch arise the posterior cornu and body of 

 the hyoid bone. 



In the embryo of mammalian animals, the fourth arch is very 

 indistinct. 



Development of the Nervous System. Eeichert has shown that, 

 in their earliest condition, the central organs of the nervous sys- 

 tem are composed of two laminae united in the middle line, so as 

 to form a central groove. This groove soon becomes converted 

 into a canal, except in the position corresponding to the medulla 

 oblongata. In front of this, certain vesicles appear, from which 

 the several parts of the brain are subsequently developed. 



These vesicles have been named Prosencephalon, Deutencephalon, 

 Mesencephalon, and Epencephalon, by Professor Owen. Of these 

 vesicles, the latter, which corresponds to the cerebellum, is at 

 this early period the largest of the four. The mesencephalon, or 

 vesicle of the corpora quadrigemina, corresponds to the large optic 

 lobes in fishes, reptiles, and birds, which in these classes are only 

 two in number (corpora bigemina), and in the adult human brain 

 is represented by the small corpora quadrigemina (anteriorly nates, 

 posteriorly testes). In front of this vesicle is a small one, which is 

 formed before any of the others, and for some time is the most 

 anterior of all. This is the vesicle of the third ventricle, and con- 

 tains the optic thalami. These points are all well seen in the fish's 

 brain. 



The prosencephalon, from which the cerebral vesicles are formed, 



