Dec. II, 1873] 



JVA TURE 



109 



precedes, in all vertebrate animals, the development of the 

 backbone, making its appearance beneath the primitive 

 groove. 



The vertebra are shaped Uke rings, and enclose within 

 their circuit the spinal marrow upon which, as it were, 

 these rings are strung. From the side of each ring 

 (except at the two ends of the backbone) there juts out 

 a bony prominence called a " transverse process," and to 

 a certain number of these a bony " rib " is in most verte- 

 brate animals attached (though there are none in the 

 frog), often extending round to j )in the breast-bone in 

 front, and being capable of more or less motion, so as (by 

 their simultaneous movement) to be able to enlarge or to 

 contract the cavity of the chest, which they thus enclose 

 and protect. 



That part of each vertebra which is placed next the 

 body cavity is generally the thickest part, and is called 

 the "body," or "centrum." The series of bodies (or 

 centra) occupy the position which was at first filled by the 

 primitive notochord, the rest of the vertebral rings having 

 been formed in the sides and roof of the canal formed 

 by the upgrowth and union of the two sides of the primi- 

 tive groove of the embryo. 



The frog order is distinguished amongst vetebrates as 

 that which has the absolutely smallest number of joints 

 in the backbone. In the frog there are but nine in the 

 front of the coccyx. In the Pipa toad there are but seven, 

 the eighth vertebra (to the transverse processes of which 

 the haunch bones are attached) having become solidly 

 joined in one bone with the coccyx. 



In all higher vertebrates, i.e. in all beasts, birds, and 

 reptiles, the head is supported on an especially ring-like 

 vertebra which— because it so supports — is called the 

 atlas, and this (in almost all) can turn upon a peculiar 

 vertebra termed (from this circumstance) the axis, and 

 provided with a toothlike ipdontoiif)* process, round which, 

 as round a pivot, the " atlas " works. Nothing of the kind 

 exists in any fish. 



In the frog (and in all its class) we find but a single 

 vertebra representing these two, but in some allied forms, 

 e.g. in Ainpkii/ina, this vertebra develops a median pro- 

 cess, reminding us of the odontoid process of the axis. 



The frog, as has been said, has no ribs, in spite of the 

 long "transverse processes" which project out on each 

 side of the backbone. Ribs are not necessary to it, for it 

 could apply them to none of the purposes to which ribs 

 are ever applied. 



In all beasts ribs aid importantly in respiration, serving 

 by their motions alternately to inflate or empty the lungs 

 by enlarging or contracting the cavity of the chest in the 

 way before mentioned. The frog, however, breathes ex- 

 clusively, as regards the lungs, by swallowing air by a 

 mechanism which will be described shortly. 



In serpents the ribs are the organs of locomotion, as 

 also in the Flying Dragon before referred to ; but in frogs 

 locomotion is effected exclusively by the limbs. In the 

 very aberrant species, I'ipa and Dactylethra there are on 

 each side of the anterior parts of the body two enormously 

 long transverse processes, each process bearing at its ex- 

 tremity a short flattened, straight osseous or cartilaginous 

 rib. These little ribs can, however, take no part in such 

 functions as those just referred to. 



Ribs, moreover, are found in the other existing orders 

 of the frog's class, i.e. both in the Urodela and Ophio- 

 viorplia. In none, however, do they join a breast-bone, or 

 sternum, another character in which Batrachians agree 

 with fishes, though they difter from fishes in that they have 

 a sternum at all. In ascending from fishes through the 

 vertebrate sub-kingdom, a sternum first appears in the 

 class Bratrachia. 



In a certain North African .Salamander named Pleu- 

 rodclcs the ribs are not only elongated, but their apices, 



* Trom ucoi'G a tootb, .ind u.iric, form. 



if they do not actually perforate the skin, are so prominent 

 as to seem to do so when the finger is drawn from behind 

 forwards along the side of the animal's body. 



The several joints of the backbone are connected to- 

 gether by surfaces which are not the same on both the 

 anterior and posterior sides of the centrum, or body, of 

 the same vertebra. Each o< the first seven vertebrae is 

 furni hed with a round prominence, or head, on the 

 hinder side of its centrum, and each of the p ecoccygeal 

 vertebrae, except the first and last, has the anterior surface 

 of its centrum excavated as a cup for the recepion of the 

 ball of the hinder surface of the vertebra next in front. 

 The first vertebra has in front two concavities, side by 

 side, to articulate with the skull. The eighth vertebra has 

 a concavity at e ich end of its " body." The ninth vertebra 

 has a body provided with a single convexity in front and 

 a double convexity behind, to articulate with the con- 

 cavities placed side by side on the front end of the coccyx. 



These arrangements are not constant in the frog's 

 order, still less in its class. In Bombiuater and Pipa the 

 vertebrae are concave behind each centrum, instead of in 

 front : and the same is the case in Salamandra. In 

 many tailed Batrachians the vertebra; are biconcave, as 

 e.g. in Spclcrpcs, Ainphimiia, Proteus and Siren. 



The biconcave shape is an approximation towards the 

 condition which is almost universal in bony fishes, though 

 not quite universal, since the bony pike {Lepidosteus) has 

 a ball at one end of each vertebra and a cup at the other. 

 Moreover, even in some reptiles {e.g. the lizards called 

 Gecl'oes) the vertebra: are biconcave, and the same was 

 the case with the majority of those species of crocodiles 

 the remains of which are found in strata older than the 

 chalk, and even in existing crocodiles the first vertebra of 

 the tail is biconcave. 



Vertebra; with a cavity in front of the centrum and a 

 ball behind it are found in the crocodiles now living as well 

 as in the frog, while vertebrae with a ball in front and a 

 concavity behind are found even amongst beasts, as in 

 the joints of the neck of Ruminants, e.g. the sheep. Thus 

 though the vertebra; of the frog's class exhibit no very 

 decided signs of affinity, they show more resemblance to 

 those of fishes than to those of any other non-batrachian 

 class. 



The transverse processes of the ninth or last vertebra 

 in front of the coccy.x, articulate with the haunch 

 bones, but are not very remarkable in shape. In some 

 frogs and toads the transverse processes of this vertebra 

 become enormously expanded, and the expanded or 

 non-expanded condition of this part is a character 

 made use of in zoological classification. The coccyx 

 is made up mainly, as has been said, of a continuous 

 ossification of the sheath of the notochord, and never 

 consists of distinct vertebra. Nevertheless, the small 

 bony arches which are at first distinct coalesce with it. 

 These arches are called " neural ' because they arch over 

 the hinder part of the spinal marrow. The great nerve 

 of the leg (the sciatic nerve) proceeds outwards on each 

 side through a foramen situated at the anterior end of the 

 coccyx from the spinal marrow — the spinal marrow being 

 that structure which gives origin to the great mass of the 

 nerves pervading the entire frame. 



St. George Mivart 



{To be continued) 



[The author sincerely regrets, that by an inadvertence for 

 I which he is exclusively answerable, two cuts introduced into 

 the second of these articles, namely, the figures representing 

 Hina esculcnta and Bufovulgaris, were copied, without sanction, 

 from two illustrations in Professor Bell's "History of British 

 Reptiles," published by Rfr. \^t\ Voorst, to whom, therefore, 

 this apology is due.] 



