ON THE VERTEBRATE SKELETON. 29/ 



of the haemal arch of the nasal vertebra with that of the frontal vertebra, 

 and consisting of the two styliform ossicles (20 and 27) which extend from the 

 haemapophysis, 21, 21", to the pleurapophysis, 28 : the essential relationship of 

 the compound ray, 26 and 27, with the nasal vertebra, is indicated by their 

 becoming confluent with its haemapophysis (at 22"), whilst they always main- 

 tain an arthrodial articulation with the pleurapophysis (2s) of the succeeding 

 vertebra. 



The bones of the splanchno-skeleton intercalated with the segments of the 

 endoskeleton in the bird's skull are the petrosal (ie), between the neural 

 arches of the occipital and parietal vertebra, early coalescing with the ele- 

 ments of those vertebrae with which it comes in contact : the sclerotals (17), 

 interposed between the frontal and nasal neural arches : and the thyro-hyals 

 (47), retained in connection with the debris of the hsemal arch of the parietal 

 vertebra, H n. The olfactory capsule remains cartilaginous. The dermal 

 bone (73) is well-developed and constant : a second supraorbital dermal bone 

 is occasionally present. All the endoskeletal bones manifest, under every 

 adaptive modification, the segmental arrangement, and it is difficult to con- 

 template the repetition of the arrangement of the cranial bones around the 

 primary segments of the encephalon in the series of arches closed respectively 

 by the bones Ni, N n, N in, N iv, together with that of the corresponding 

 number of arches closed below, at H iv, H in, H u and H i, without a con- 

 viction that the type illustrated in fig. 15 is that upon which these seg- 

 ments of the skull have been constructed. This conclusion might seem 

 forced, in respect to the occipital vertebra, were its displaced hsemal arch 

 and appendages to be viewed without reference to their relative position and 

 connections in the lower vertebrate classes ; but it will be confirmed and 

 shown to be agreeable to nature and to the recognised kinds and grades of 

 modification to which the elements of one and the same vertebra are subject, 

 by observing in the young bird the distinct pleurapophysial elements of those 

 cervical vertebrae, beyond which the corresponding elements of the occiput 

 have retrograded, in obedience to the functions which the haemal arch of 

 that vertebra and its appendages are destined to perform in the feathered 

 class. 



Mammals. — If the foregoing views of the general homology of the bones 

 of the skull be agreeable to their essential nature, we should expect that the 

 new and additional modifications, in the mammalian class, which tend to 

 obscure those relations would be seated in the appendages and peripheral 

 elements of the endoskeletal segments, or in the capsules and appendages of 

 the special organs of sense. 



I have selected with a view to testing such anticipation the skull of a young 

 pachyderm*, and, after successively disarticulating the segments in the order 

 in which they have been previously described, I have given a side view of 

 them arranged in correspondence with the figures 23, 22, and 5. (Fig. 24.) 



The neural arch of the occipital vertebra, Ni, agrees with that of the bird and 

 crocodile in the coalescence of the par apophysis, 4, with the neur apophysis, 

 2 ; but the process, 4, now descends from the lower part of the arch, and, 

 as in many other mammals, is of great length. An articular condyle is also 

 developed from each neurapophysis which articulates with the concave an- 

 terior zygapophysis of the atlas, and is the homotype of the posterior zyga- 

 pophysis in the trunk-vertebrae. The centrum (1) is reduced, like that of 

 the atlas, to a compressed plate, and its hinder articular surface is not more 



* The skull of the ruminant is perhaps still batter adapted to demonstrate the vertebral 

 relations of the cranial bones : that of the sheep is the subject of the diagram for this pur- 

 pose in the concluding volume of my' Hunterian Lectures.' 



1846. • x 



