302 ANATOMY OF VERTEBRATES. 



fig. 81. And, if the interpretation of the more normal or archetypal 

 condition of this segment in the lower Vertebrate animal, fig. 

 101, A, Vol. I., be accepted, so also must be the explanation here 

 given of the nature of the modifications of the special homologues 

 of the constituents of the occipital segment by which that arche- 

 type is masked in the Mammal. A single nerve supplies the 

 appendage, 53-57, in Protojjterus ; subsequent developement of that 

 appendage in higher forms presses more nerves from other centres 

 into its service ; these do not originate the complex conditions 

 calling for them. And if the simple limb, fig. 101, A, 53-57, be 

 the special homologue of the complex one, fig. 189, 53-57, neither 

 the number of nerves, of vessels, or of terminal rays can afiect 

 the conclusions deducible from fig. 101, as to its general nature 

 in relation to the Vertebrate archetype. 



In the second segment of the skull, Nii, the centrum, 5, is long 

 distinct from both i and 9; and the haemal arch (hyoid bone) 

 retains its natural connection with the rest of the segment, and 

 by means of a more complete developement of the pleurapophyses, 

 38, than in any of the inferior air-breathing Vertebrates. In the 

 Hog, as in other Mammals, may be separated, without artificial 

 division of any compound bone, the entire parietal segment, but 

 with it is brought away the petrified capsule of the acoustic organ 

 and the anchylosed distal piece, 27, of the maxillary appendage, 

 which more or less conceals the typical character of the neural 

 arch of the parietal vertebra in every Mammal : least so, however, 

 in the Monotremes and Ruminants. The neurapophyses, 6, of the 

 parietal vertebrae have coalesced with the centrum, 5, but retain 

 much of the proportions they present in the cold-blooded classes ; 

 for the mesencephalic segment of the brain is, in fact, but little 

 more developed in the Mammal : they are notched in the present 

 example, but are perforated in the Sheep, by the larger divisions 

 of the trigeminal, and they send down an exogenous process, 

 which articulates and sometimes coalesces with the appendage, 2 1, 

 of the palato-maxillary arch, and with the pleurapophysis, 20, of 

 the same arch. The neural spine, 7, always developed from a 

 pair of centres in Mammals, often vastly expanded, and sometimes 

 complicated with a third, intercalary or interparietal osseous piece, 

 in subserviency to the large size of the prosencephalon, is occa- 

 sionally uplifted and removed from the neurapophyses by the 

 interposed squamous expansion of the bone, 27 ; but this, which 

 reminds one of the occasional separation of the neural arch from 

 the centrum of the atlas in Fishes, is a rare modification in the 

 Mammalian class. The diapophysis, 8, always commences as an 



