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° F., and the other extreme of 

 cold retards the activity of living animals, and even destroys them alto- 

 gether. The salamanders, however, are capable of sustaining a con- 

 siderable degree of cold without its having any effect upon the system. 

 Thus Notopthalmtis 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 Scincidce, have only 

 two hind feet, while others, as tSiren, 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 Jhat 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 perennihrnnchiate hatrachians, as Proteus, Mcnohranchus, 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. 



icoxoGKAnnc ENTYCi.oiM5r)iA.— voi.. ir. 5^9 449 



