DEVELOPMENT OF THE SKULL. 



955 



brought into connection with them, so as to form the iuclosure of the vis- 

 ceral cavity (Fig. 356, B). From the pleurapophyses are occasionally de- 

 veloped a pair of " diverging appendages " (o, a), which are well seen in the 

 ribs of Birds; and these are considered by Prof. Owen to be the funda- 

 mental elements of the bones of the " extremities " or " limbs," those of the 

 anterior extremity being on his view the diverging appendages of the occip- 

 ital vertebra, and those of the posterior extremity standing in the same 



FIG. 356. 



Elements of a Vertebra according to Prof. Owen : A, ideal typical vertebra ; B, actual thoracic verte- 

 bra of a Bird ; c, centrum, giving off d, d, the diapophyses, and />,;>, the parapophyses ; the neural arch, 

 inclosing the spinal cord, * is formed by ra, n, the neurapophyses, and ns, the neural spine; the htenial 

 arch, inclosing the great centres of the circulation, is formed by h, h, (he hsemapophyses, and fis, the 

 hsemal spine. From both the neurapophyses and hffimapophyt-es may be given off the zygapophyses, 

 z, z. The lateral arches, which may inclose the vertebral arteries, o, o, are completed by the pleur- 

 apophyses, pi; these in B are bent downwards, so as to form part of the haemal arch, and give oft' the 

 diverging appendages, a, a. 



relation to one of the sacral vertebrae. The number of the segments analo- 

 gous to Vertebrse entering into the skull has been a subject of much discus- 

 sion among those who adopt the " vertebral theory " of its composition ; but 

 Prof. Owen agrees with Goethe and Oken (the original propouuders of that 

 theory) in fixing the number at four, which corresponds with the points of 

 ossification, succeeding each other in a linear series, that, though absent in 

 the earlier, are apparent in the later stages of the development of the En- 

 cephalon, namely (proceeding from behind forwards), the Encephalon, the 

 Mesencephalon, the Proseucephalon, and the Rhinencephalon (Fig. 362); 

 and also corresponding with the number of the nerves of special sense, the 

 Auditory, Gustative, Optic, and Olfactory, which issue from this part of the 

 neural axis with the same segmental regularity that the ordinary sensori- 

 motor nerves do elsewhere. 



794. In the development of the Skull the same three stages may be traced 

 as that in the Vertebral column, namely, the membranous, the cartilaginous, 

 and the osseous ; the result of the first two forming what is now called the 

 Primordial Cranium. In the first stage, the membrane of the Cranium is 

 formed from the anterior part of the prevertebral plates (uwh, Fig. 353), 

 which shoot forwards in front of the pointed extremity of the Chorda dor- 

 salis or Notochord, and throw out processes on either side, that, arching up- 

 wards, ultimately form a case for the brain. An essential difference from 



