14 INTRODUCTORY LECTURE. 



Reptiles are cold-blooded, like Fishes ; but all of tliera possess 

 lungs, or organs for breathing attnospheric air. Most of the class 

 exercise the function of these organs ; but some, retaining gills, 

 chiefly breathe water ; and those with lungs alone ai'e less dependent 

 on respiration than the higher Vertebrata. Hence the Reptiles were 

 defined by Linnaius as " arbitrary breathers," — '•'■ Pulmones spirantes 

 arbitrarie,'" — and were called by him ^^ Amphibia." The blood is 

 remarkable for the large relative size and constant elliptical form of 

 its red particles {Jig. 4. d, e), which, as in Fishes, have a distinct 

 granular nucleus. And, what is more remarkable, the size increases 

 in the ratio of the persistence of the branchial organs. You may, for 

 example, discern the blood-discs with the naked eye in the Siren 

 lacertina {fig. 4./). The typical condition of the heart in Reptiles 

 is three-chambered ; having two auricles and one ventricle ; one 

 auricle receives the venous blood from the general system, the other 

 that which has undergone chemical change in the lungs : both kinds 

 of blood are mixed in the ventricle, and distributed in that state, 

 partly to the lungs again, partly to the general system. The breath- 

 ing apparatus is so far inferior to that of fishes, as that the whole 

 mass of circulating fluid is not distributed through it ; but this appa- 

 rently retrograde step in development seems as if preparatory to the 

 establishment of a more perfect respiratory system, adapted to the 

 exigencies of higher classes of animals. Always, however, in using 

 or hearing this metaphorical language, it is to be borne in mind, that 

 each condition, which represents a step in progress as regards the 

 series of species, is complete and perfect in relation to the particulai 

 species in which it is manifested. 



The nervous system of Reptiles presents an advance in the 

 larger proportional size of the cerebral lobes ; but the whole brain 

 is still a mei"e linear series of smooth ganglionic masses, and 

 the cerebellum is often inferior, in size and complexity, to that in 

 Fishes. 



The eyes are smaller than in Fishes, but generally more perfect 

 and defended by eyelids : the ears are provided with a vibratory 

 membrane and chamber, called the "tympanum:" but the most 

 characteristic feature of Reptiles in contrast with Fishes, which 

 the organs of the senses present, is the establishment of a communi- 

 cation from the eye, the ear, and the nose respectively, with the 

 respiratory tract or mouth ; the eye by the lachrymal duct, the ear 

 by the Eustachian tube, and the nose by its prolongation into 

 a meatus, with a posterior opening into the mouth, or fauces. 

 This latter character the Siren manifests, but not the Lepidosiren, 

 nor any true Fish. The sense of touch must be enjoyed by the naked 



