FACE AND VISCERAL ARCHES. 595 



have united inferiorly. Those vertebrae which do not bear 

 ribs, such as the cervical vertebrae, have generally an additional 

 centre of ossification in the transverse process, which is to be 

 regarded as an abortive rudiment of a rib. In the foetal bird, 

 these additional ossified portions exist in all the cervical ver- 

 tebrae, and gradually become so much developed in the lower 

 part of the cervical region as to form the upper false ribs of 

 this class of animals. The same parts exist in mammalia and 

 man ; those of the last cervical vertebrae are the most developed, 

 and in children may, for a considerable period, be distinguished 

 as a separate part on each side, like the root or head of a rib. 

 The true cranium is a prolongation of the vertebral column, 

 and is developed at a much earlier period than the facial bones. 

 Originally, it is formed of but one mass, a cerebral capsule, 

 the chorda dorsalis being continued into its base, and ending 

 there with a tapering point. This relation of the chorda dor- 

 salis to the basis of the cranium is persistent through life in 

 some fish, e. g., the sturgeon. The first appearance of a solid 

 support at the base of the cranium observed by Miiller in fish, 

 consists of two elongated bands of cartilage, one on the right 

 and the other on the left side, which are connected with the 

 cartilaginous capsule of the auditory apparatus, and united 

 with each other in an arched manner anteriorly beneath the 

 anterior end of the cerebral capsule. Hence, in the cranium, 

 as in the spinal column, there are at first developed at the 

 sides of the chorda dorsalis two symmetrical elements, which 

 subsequently coalesce, and may wholly inclose the chorda. 1 



Development of the Face and Visceral Arches. 



It has been said before that at an early period of develop- 

 ment of the embryo, there grow up on the sides of the primi- 

 tive groove the so-called dorsal lamince, which at length 

 coalesce, and complete by their union the spinal canal. The 

 same process essentially takes place in the head, so as to in- 

 close the cranial cavity. 



The so-called visceral lamince have been also described as 

 passing forwards, and gradually coalescing in front, as the 

 dorsal laminae do behind, and thus inclosing the thoracic and 

 abdominal cavity. An analogous process occurs in the facial 

 and cervical regions, but the inclosing laminae, instead of being 

 simple, as in the former instances, are cleft. 



1 For much new and original matter relating to the development 

 of the cranium, the reader is referred to the important lectures on 

 Comparative Anatomy, delivered at the College of Surgeons by Pro- 

 fessor Huxley. 



