ON THE VERTEBRATE SKELETON. 261 



I am inclined to regard the ' odontoid process ' of the mammalian axis as the 

 homologue of one of these subvertebral wedge-bones, or peripheral develop- 

 ments of the outer layer of the notochordal capsule. It cannot be the pecu- 

 liarly developed anterior articular epiphysis of the second vertebra, since this 

 is represented by a distinct centre of ossification between the odontoid process 

 and the body of that vertebra, according to Professor Muller's observation 

 in a foetal foal *. 



-The diverging appendages of the haemal arch in the abdominal vertebrae of 

 fishes present the form of long and slender spines (fig. 17, « «), usually at- 

 tached to, or near the head of the ribs, and extending upwards, outwards 

 and backwards, between the dorsal and lateral portions of the muscular 

 segments, to which they afford a firmer fulcrum or basis of attachment; 

 acting, therefore, as so many pairs of rudimental and concealed limbs. They 

 are termed the 'obere rippe' by Meckel, and at the fore-part of the abdomen 

 of the polypterus they are stronger than the pleurapophyses themselves. 

 As the vertebrae approach the tail these appendages are often transferred 

 gradually, from the pleurapophysis to the parapophysis, or even to the cen- 

 trum and neural arch. 



In the air-breathing vertebrata, in which the heart and breathing organs 

 are transferred backwards to the trunk, the corresponding osseous segments 

 of the skeleton are in most instances developed to their typical complete- 

 ness, in order to encompass and protect those organs. The thoracic haemapo- 

 physes in the crocodiles are partially ossified, and in birds (fig. 15, h, A) com- 

 pletely so; in which class the haemal spines of the thorax (hs) coalesce together, 

 become much expanded laterally, and usually develope a median crest down- 

 wards to increase the surface of attachment for the great muscles of flight. 

 This speciality is indicated by the name ' sternum ' applied to the confluent 

 elements in question. The abdominal haemapophyses and spines retain their 

 primitive aponeurotic condition, though still preserving their characteristic 

 expansion f. In the crocodiles and enaliosaurs the abdominal haemapophyses 

 are also ossified; and, in the latter, they manifest the same composite character 

 which has been noticed in the pleurapophyses of the sturgeon, consisting of 

 three or more pieces, which overlap each other %. The abdominal haemal 

 spines, in the Plesiosaurus Hawkinsii, are transversely extended, they are 

 marked c c in the figure quoted below : the compound haemapophyses them- 

 selves are marked b b in the same figure. 



The typical thoracic vertebrae of most birds support diverging appendages 

 (fig. 15, a, a), either anchylosed or articulated, as in the penguin and apte- 

 ryx, to the posterior border of the pleurapophysis (pi). The function of the 

 appendages in this form of typical vertebra is to connect one haemal arch 

 with the next in succession, so as to associate the two in action, and to give 

 firmness and strength to the whole thoracic cage. (A portion of the next 

 rib so overlapped is shown at pi, a, fig. 15.) 



With regard to the connections of the pleurapophyses, we have seen that, 

 in fishes, they may be directly attached to the centrum, or to the ends of the 

 parapophyses (fig. 17,j»),or they may be quite detached from their proper seg- 

 ment, and suspended to the haemal arch of an antecedent vertebra, as in the 

 case of the clavicle or epicoracoid, no. 2s. In batrachians, ophidians, and 

 lacertians, the proximal end of the pleurapophysis is simple, as in fishes, 

 but is articulated to an exogenous tubercle or transverse process from the 



* Vergleichende Anatomie der Myxinoiden. Abhand. Akad. der Wissensch. Berlin, 

 1834, p. 105. 

 t Myology of Apteryx, Zoological Transactions, vol. iii. pt. iv. pi. 35, g*, g*. 

 % Buckland, Bridgewater Treatise, vol. ii. pi. 18, fig. 3. 



