REPTILIA. 



313 



subsequently two dorsal ganglia occur, and 

 further down, towards the middle of the back, 

 there occurs a third and last ganglion, which 

 furnishes the splanchnic nerve : the re- 

 mainder of the sympathetic is made up of one 

 or two cords, which, in the sacral region, give 

 off a great number of branches, the divisions 

 of which form the renal, hypogastric, and sa- 

 cral plexuses. 



According to Mr. Swan, in the turtle (Tes- 

 tudo Mydas), the sympathetic is free and 

 distinct throughout the whole length of the 

 neck, but it gives off filaments of intercom- 

 munication to the nervus vagus. One of its 

 branches passes along with the division of 

 the carotid artery into a canal in the base of 

 the cranium, gives off a filament to the facial, 

 and communicates with the second division of 

 the fifth pair. 



Organ of Hearing. In all reptiles the organ 

 of hearing is constructed to appreciate sounds 

 communicated through the medium of the 



the scala vestibuli and the scala tympani of 

 the human ear. In Crocodiles this cochlea is 

 of considerable size, and may be easily exhi- 

 bited in very young individuals ; it is more 

 difficult to find in the Chameleon and in 

 Lizards, whilst in Serpents it is reduced to a 

 very rudimentary condition. In Tortoises, that 

 part of the ear which seems to represent the 

 rudimentary cochlea resembles exactly that 

 part of the ear called by Cuvier the sac, both 

 in its shape, and from the circumstance of 

 its containing otolithes, and he is disposed to 

 consider this sac as truly analogous to the 

 cochlea of the human ear ; whilst that portion 

 of it which he calls the sinus, he considers as 

 representing the vestibule. 



In all reptiles the membranous labyrinth is 

 enclosed in an osseous sheath, which embraces 

 it more or less closely in different genera ; in 

 the Saurians the bony hbyrinth is complete, 

 but in Tortoises that portion which separates 

 the vestibule from the cranial cavity is not 



atmosphere, and consequently differs from ossified, but remains partially membranous. 



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that of fishes in several important particulars, 

 amongst the most obvious of which is the 

 addition of a membrana tympani and tym- 

 panic cavity, wherein is lodged an ossiculum 

 auditus, the office of which is to convey the 

 vibrations of the tympanic membrane to the 

 labyrinth contained within the skull. 



In all reptiles the internal ear consists of 

 the same parts as that of fishes, only they are 

 comparatively of smaller size and more com- 

 pact in their arrangement. In crocodiles and 

 lizards, the internal ear consists of three semi- 

 circular canals, which exhibit the usual ar- 

 rangement, each canal forming a considerable 

 portion of a circle, and presenting internally a 

 membranous ampulla before opening into the 

 vestibule. 



Towards the interior of the skull, there is 

 an organ appended to the vestibule, which is 

 evidently analogous to the sac met with in 

 the ear of fishes. The walls of this sac are 

 membranous, and copiously supplied with 

 bloodvessels. It is found to contain in its 

 interior three otolithes ; but these are very 

 small, and even of softer texture than those 

 of the chondropteriginous fishes. 



Besides the above parts, the internal ear of 

 reptiles presents for the first time an addi- 

 tional part, which is undoubtedly a rudimen- 

 tary cochlea. This is an appendage to the 

 vestibule, of a conical shape, and slightly bent 

 towards its extremity ; it lies inferior to the 

 vestibule, its apex being directed towards the 

 mesial line of the cranium. On opening this 

 organ, it is found to be divided internally into 

 two compartments by a double cartilaginous 

 septum : the two compartments communicate 

 with each other towards the apex of the cone, 

 whilst at their opposite extremities one of 

 them is found to open into the vestibule, 

 whilst the other terminates at a small orifice 

 closed by membrane, which communicates 

 with the tympanic cavity. This organ, it will 

 be perceived, is precisely comparable to the 

 rudimentary cochlea met with in the ear of 

 birds, the two canals representing respectively 



The tympanic cavity, which in reptiles is 

 for the first time interposed between the ves 

 tibule and the exterior of the body, varies in 

 its arrangement in different genera. In the 

 Crocodiles this cavity might be divided into 

 two portions, one external, which is very wide, 

 and is closed externally by the membrana 

 tympani and the skin ; and an internal portion, 

 which is separated from the former by a con- 

 striction. It is in this latter compartment that 

 the two fenestrae are situated ; and it contains, 

 moreover, some cavities which are analogous 

 to the mastoid cells, but of much larger size 

 than in mammiferous animals. The position 

 of the tympanum in this reptile is near the 

 upper part of the cranium. 



The tympanic cavity of the Chelonian 

 reptiles is situated much more laterally than 

 in the crocodile, and the constriction which 

 separates the external from the internal portion 

 is less remarkable. The internal compart- 

 ment of the tympanum is here prolonged 

 backwards into a wide rounded cell. At the 

 bottom of this cavity, just opposite the mem- 

 brana tympani, there is a narrow canal lead- 

 ing to the fenestra ovalis, in which the cssi- 

 culum auditus is lodged. The Eustachian 

 tube is a canal of moderate length, which 

 runs downwards and slightly backwards to 

 communicate with the palate just behind and 

 internal to the articulation of the lower jaw. 



In the generality of Saurian reptiles, the 

 walls of the tympanic cavity are membranous 

 posteriorly and inferiorly : the Eustachian 

 canal is very short, and opens into the palate. 

 In many genera the tympan'c bone is much 

 enlarged superiorly, so that the cavity of the 

 tympanum is made more extensive : this ar- 

 rangement is most conspicuously seen in the 

 genus Draco. 



In Serpents there cannot, strictly speaking, 

 be said to be any tympanic cavity, the handle 

 of the auditory ossicle being imbedded amongst 

 the flesh, so that its extremity only touches 

 the skin close behind the articulation of the 

 lower jaw. 



