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PHYLUM CHORDATA 



349 



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arches cross one another, and where their walls are in contact 

 is an aperture -the foramen Panizzce placing their cavities in 

 communication. 



The brain of Reptiles is somewhat more highly organised than 

 that of the Amphibia. The brain substance of the cerebral 

 hemispheres exhibits a distinction into superficial grey layer or 

 cortex containing pyramidal nerve-cells, and central white medulla, 

 not observable in lower groups. The cerebral hemispheres are 

 well developed in all, and there is a hippocampus (see below in 

 the description of the brain of the Rabbit, and of that of the 

 Mammals in general) in the shape of a specially modified region 

 of the dorsal and mesial walls of each hemisphere, represented 

 less distinctly in the Amphibia ; a commissure the hippocampal 

 connects the hippo- 

 campi of opposite sides, 

 and is dorsal to the 

 chief cerebral commis- 

 sure the anterior com- 

 missure. The mid-brain 

 consists dorsally usually 

 of two closely-approxi- 

 mated oval optic lobes ; 

 rarely it is divided 

 superficially into four. 

 The cerebellum is always 

 of small size, except in 

 the Crocodilia (Fig. 

 1015), in which it is 



Comparatively highly Fio 1014 _ Heart of crocodile with the principal arteries 

 developed, and Consists (diagrammatic). The arrows show the direction of the 



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OI a median and tWO 



lafpral lr>T-ip 

 '" S- 



SenSOrV Organs. _ 

 J V,. ," 



111 most Lacertllia, but 



not in the Ophidia, the 

 nasal cavity consists of two parts an outer or vestibule, and 

 an inner or olfactory chamber the latter having the sense-cells 

 in its walls, and containing a turbinal bone. In the Turtles each 

 nasal chamber is divided into two passages, an upper and a lower, 

 and the same holds good of the hinder part of the elongated nasal 

 chamber of the Crocodilia. 



Jacobson's organs (Fig. 983) are present in Lizards and Snakes, 

 absent in Chelonia and Crocodilia in the adult condition. 



The eyes are relatively large, with a cartilaginous sclerotic in 

 which a ring of bony plates (Fig. 984) is developed in some cases. 

 The muscular fibres of the iris are striated. A pecten is present 

 in most. Most Reptiles have both upper and lower eyelids and 



arterial and venous currents. I. aort. left aortic arch ; 

 ;. aur . left auricle ; I. am. vent. ap. left auriculo-ventri- 

 cular aperture ; I. car. left carotid ; /. sub. left subclavian ; 

 1. vent, left ventricle ; pul. art. pulmonary artery ; r. aort. 

 right aortic arch ; r. aur. right auricle ; r. aur. vent. ap. 

 right auriculo-ventricular aperture ; r. car. right carotid ; 

 r. sub. right subclavian ; r. vent, right ventricle. (From 



