June 1, 1894.] 



K«N O W L E D G E . 



189 



breathe in the ordinary way by expanding and contracting 

 its chest, and therefore has to swallow air by taking in a 

 gulp, closing the mouth, and then forcing it into the 

 lungs. 



Regarding, then, a serpent as essentially a " rib-walker," 

 we have to see, in the first place, how this most peculiar 

 and original mode of progression is effected ; while in the 

 latter portion of our article we must point out the chief 

 family groups into which serpents are divided, and likewise 



Front, Back, ami Left Side of the \'ert«br,i of a Suake 



indicate why naturalists will run contrary to popular 

 opinion in telling us that a blind-worm is a lizard and not 

 a snake. 



In a creature that has taken to a mode of progression so 

 peculiar as is that of a snake, the first essential is a 

 combination of strength with extreme flexibility in the 

 backbone, and this is attained by the development of an 

 extremely complicated system of articulation between its 

 constituent vertebras. In ordinary terrestrial vertebrates 

 the vertebrfe are connected with one another by two main 

 articulations. In the first place, the centrum or body of 

 every vertebra has a surface at each end for articula- 

 tion with the adjacent members of the series, and when 

 firmness and strength is the main object, as in the back 

 of mammals, these two surfaces are nearly fiat ; when, 

 however, greater mobility is required, as in the neck of 

 the more specialized hoofed mammals, this is efiected by 

 means of a ball-and-socket joint ; and in the snakes this 

 baU-and-socket arrangement extends throughout the whole 

 series, the cup {c of the foregoing figure) being in 

 front, and the ball (c') behind. In addition to this articu- 

 lation, vertebrfe are ordinarily connected by what are 

 termed zygapophyses, or flattened facets attached to the 

 arch or upper half of the structure — the anterior zygapo- 

 physis {:e) always looking upwards, while the hinder one 

 (ze) faces downwards. For all ordinary purposes these 

 two systems of articulation are amply suflicient ; but 

 snakes required something more, and they have accordingly 

 developed an additional pair. In the upper part of the 

 middle figure of our illustration there will be seen a pair 

 of deep pits (ci) on the hinder aspect of the arch of the 

 vertebra, which have received the somewhat long name of 

 zygactra, and show a flattened articular facet directed 

 upwards and inwards on the lower half of their outer sides. 

 Into each of these pits is received a somewhat wedge-shaped 

 projection from the front of the arch of the adjacent ver- 

 tebra (marked :i in the left-hand figure of our illustration), 

 and termed the zygosphene, the flattened articular facet 

 of which looks downwards and outwards. Such a triple 

 system of articulation produces, of course, extreme flexi- 

 bility of the backbone, without any danger of dislocation of 

 the joints, as any of our readers may ascertain for them- 

 selves by handling the articulated skeleton of a snake. It 



might be thought that this remarkable system of articulation 

 was peculiar to snakes, and if it were so we should be able 

 to define that group of animals much more neatly and 

 definitely than we are at present able to do. Unfortunately, 

 however, for the systematist, the iguanas among living 

 lizards, together with certain members of some extinct 

 groups of reptiles, have precisely similar vertebral articu- 

 lations, which are due to that tiresome parallelism in 

 development alluded to so frequently in some of our earlier 



articles. We believe, however, 

 we are right in saying that 

 none of the snake-like lizards 

 possess these additional arti- 

 culations to the vertebrfe. 



Before leaving the subject 

 of the backbone, it must be 

 added that the ribs, which are 

 single-headed, are attached 

 to a facet (</) on the lower part 

 of the front end of each side 

 of the body of the vertebra ; 

 such facets being usually 

 situated on the body itself, 

 although in the figured speci- 

 men, which belongs to an ex- 

 tinct genus, they have distinct 

 pedicles. Although this mode of rib-connection is common 

 to lizards and serpents, it is precisely that best adapted 

 to afl'ord the greatest amount of motion to " rib-walkers." 

 It may be added that in all snakes the breast-bone and 

 bones of the shoulder, Uke those of the fore limb, are 

 completely wanting ; while there are, at most, only 

 vestiges of those of the pelvis and hind limbs. 



Dismissing the interesting subject of the skeleton with 

 these few remarks, we come to the consideration of how 

 the reptiles under consideration walk. Now, if we 

 examine any ordinary snake, we shall find that the under 

 surface is covered with a series of oblong transverse 

 shields, overlapping one another by their hinder edges, 

 and in the body extending right across the lower surface, 

 although in the tail they very frequently form a double 

 series. Each of these shields corresponds to a pair of 

 ribs, and all are of the utmost importance in terrestrial 

 locomotion. On this point Dr. Giinther writes that when 

 any portion of the body of a snake " has found some 

 projection on the ground which affords it a point of 

 support, the ribs, alternately of one and the other side, 

 are drawn more closely together, thereby producing alter- 

 nate bends of the body on the corresponding side. The 

 hinder portion of the body being drawn after, some 

 part of it finds another support on the rough ground or a 

 projection ; and the anterior bends being stretched in a 

 straight line, the front part of the body is propelled in 

 consequence. During this peculiar kind of locomotion 

 the numerous hard shields of the belly are of great advan- 

 tage, as, by means of their free edges, they are enabled to 

 catch the smallest projections on the ground, which may 

 be used as points of support.' It follows from this that 

 on a perfectly smooth surface, like a sheet of glass, a 

 snake is utterly unable to move. It may be added that 

 these reptiles can only progi-ess by imdulating the body in 

 a horizontal direction ; and that the pictures sometimes seen 

 of snakes with the body thrown iato folds in a vertical 

 plane are pure evolutions of the artistic imagination. 



That the large inferior shields of snakes have been 

 specially developed for the needs of this peculiar mode of 

 progression, there can be no doubt ; but, although they 

 are absent in the snake-like lizards, they are by no means 

 characteristic of the group as a whole. A further 



