REPTILIA. 245 
as to prevent the access of blood to these organs, yet the animal appeared 
to experience but little inconvenience. 
All Reptilia are cold-blooded, that is, are not able to maintain a uniform 
temperature, this being regulated, within certain limits, by that of the 
external air. Variations of external temperature, however, exercise a 
great influence upon the functions of these animals. Many species are 
sensibly affected by a temperature of 120° I°., and the other extreme of 
cold retards the activity of living animals, and even destroys them alto- 
gether. The salamanders, however, are capabie of sustaining a con- 
siderable degree of cold without its having any effect upon the system. 
Thus Notopthalnus viridescens has been seen frozen up in ice, yet exhi- 
biting a considerable degree of activity when liberated. In many reptiles 
torpidity ensues upon a certain reduction of temperature. 
Most reptiles possess the four typical vertebrate extremities, two anterior 
and two posterior, serving either for running, leaping, or swimming. 
These, however, are not characterized by external development, being 
generally short in proportion to the rest of-the body, so that the belly 
either drags along the ground, or nearly touches it. The Ophidia are, 
however, destitute of limbs, either entirely, or possess them only in a 
rudimentary state. Other forms again, as among the Scincide, have only 
two hind feet, while others, as Siren, possess the anterior pair alone. 
There is a much greater difference in the skeleton of different forms of 
Reptilia than among birds and mammalia. The bones are characterized, 
microscopically, by the cellular structure and the almost entire absence of 
central cavities. The cranium is exceedingly small in proportion to the 
entire head. The skull, on examination, will be found to exhibit more pieces 
than that of the mammal or bird, owing to the fact that fusion among the 
individual elements does not take place to anything like the extent 
observed in the classes just mentioned. Certain bones of the mouth, as 
the sphenoid and vomer, are armed with teeth; a condition of things 
which does not again recur, although existing in fishes. In the scaly 
reptile the articulation of the vertebral column with the head is by means 
of a single occipital condyle placed below the foramen. This is spherically 
convex, and produced by the combination of the basi- and ex-occipitals. 
In the naked forms, however, the basi-occipital retreats from this position, 
and the single condyle is divided into two, one on each side. These thus 
represent mammalia in their double condyles, while the squamiferous forms 
resemble birds and fishes, in having a single occipital articulation between 
the skull and vertebral column. 
The vertebral column of the reptiles is highly characteristic of the class. 
In most recent adult forms the articulations are spherically convex at one 
extremity, and spherically concave at the other. The dried skeletons of 
some of the perennibranchiate batrachians, as Proteus, Menobranchus, as 
well as of the young caducibranchiates, exhibit the biconcave structure of 
the fish vertebra. This, however, is only an apparent deviation from the 
law, as in most instances it will be found that the gelatinous ball, representing 
the convex part of the articulations, has dried up, and thus disappeared. 
ICONOGRAFHIC ENCYCLOPA&DI4.— VOL. II. 29 449 
