GENERAL CHARACTERS. 



on the ribs, termed uncinate processes, of which the position is shown 

 in the figure of the skeleton of the Eagle given below (fig. 1 106). 



The breast-bone or sternum (figs. 817, 819) is usually composed 

 of a median series of bones or cartilages on the ventral aspect of the 

 body, which is divisible in the higher groups into an anterior pre- 

 sternum, usually consisting of 

 a single ossification ; and of a 

 series of mesosternal elements, 

 followed posteriorly by the 

 xiphisternum. In many Saur- 

 opsida the sternal bones have 

 long lateral processes ; and the 

 ossifications in this class may 

 consist of a pair of bones 

 united by cartilage. 



The skull, or anterior ter- 

 mination of the axial skeleton, 

 now claims our attention, but 



of an 



Fig. 820. — Left lateral view of the skul 

 embryo Dog-fish, tr, Left trabecula ; n, o, a, 

 Nasal, orbital, and auditory capsules ; m, hy, br, 

 Mandibular, hyoid, and branchial arches ; cl, v, 

 Hyomandibular and first branchial clefts. (After 

 Parker.) 



the description of this impor- 

 tant and difficult part must 

 be of the briefest. The re- 

 searches of embryologists have shown that the skull is only a 

 special modification of the primitive elements from which the rest 

 of the axial skeleton were formed, although it does not consist, as 

 was once thought, of a series of modi- 

 fied vertebras. The skull is divisible 

 into a dorsal, or cranial, and a ven- 

 tral, or 'visceral, portion ; the former, 

 originating from a series of primitive 

 segments {somites), encloses the brain- 

 cavity ; while the latter, which has a 

 segmentation of distinct and later 

 origin, is primitively connected with 

 the function of respiration. 



The earliest commencement of the 

 primitive cartilaginous cranium occurs 

 in the formation of a pair of rod-like 

 trabecule (fig. 820), lying at the base 

 of the brain, of which the posterior 

 parachordal parts embrace the ex- 

 tremity of the notochord (fig. 821). 

 These parachordals soon unite to form 

 a basilar plate supporting the brain 



(fig. 821); while the anterior prochordal parts unite in front to 

 enclose a space (ibid., ol) for the passage of the olfactory nerves, 



Fig. 821. — Upper view of a later 

 embryo, ct, Cornu of trabecular ; pf 

 and ptf, Preorbital and postorbital pro- 

 cesses of do. ; s, Ethmonasal septum ; 

 b, Basilar plate ; no, Notochord ; ol, 

 Olfactory foramen. Other letters as in 

 fig. 820. 



