552 



of the occipital spine, 3, has been governed by the superior development of the cerebellum 

 in the mammalian class. 



The haemal arch of the occipital vertebra is not here preserved : it is represented, like those 

 of the cervical vertebrae, by the pleurapophysial elements only ; but these are developed into 

 broad triangular plates with outstanding processes : that called ' spine ' and ' acromion ' is 

 exogenous ; but that called ' coracoid ' is always developed from an independent osseous 

 centre, which is a rudimental representative of the haemapophysis, coalesces with the pleur- 

 apophysis in mammals, and, in that class, only attains its normal proportions, completing 

 the arch with the haemal spine (episternum), in the monotremes. The diverging appendage 

 (fore-limb) of this arch, though retaining the general features of its primitive radiated form, 

 has been the seat of great development and much modification and adjustment of its different 

 subdivisions in relation to the locomotive office it is now called upon to perform. 



With the exception of this excess of development of the appendage, the defective develop- 

 ment and displacement of the haemal arch, and the coalescence of the parapophyses in the 

 neural arch, there are few points of resemblance which are not sufficiently salient between the 

 segment represented by the bones, i, 2 and 3, in the mammal, and that so marked in the fish 

 (No. 148). And, if the interpretation of the more normal condition of this segment in the 

 lower vertebrate animal, according to the archetypal vertebra (Nos. 28, 29, 30), 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 archetype is masked 

 in the mammal. 



In the second segment of the skull, the centrum, s, is naturally distinct, as in other mammals, 

 and the haemal arch (hyoid bone) retains its natural connection with the rest of the segment, 

 and by means of a more complete development of the pleurapophyses (39) 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 (iu), and the anchylosed distal piece (27) of 

 the maxillary appendage, which more or less encumbers and conceals the typical character of 

 the neural arch of the parietal vertebra in every mammal : least so, however, in the mono- 

 tremes and ruminants. The neurapophyses (e) 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 mam- 

 mal : 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 (23) of the palato-maxillary arch, and with the pleur- 

 apophysis, 22, of the same arch. The neural spine (r), always developed from two 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 

 parapophysis (s) always commences as an autogenous element by a distinct centre of ossifica- 

 tion ; it speedily coalesces with the petrosal (ie), but otherwise retains its individuality in some 



