PHYLUM CHORDATA 



331 



flattened lobe overlapping the anterior portion of the metaccele. 

 The metencephalon (medulla oblongata, m. o.), broad in front, tapers 

 behind to where it passes into the anterior portion of the spinal 

 cord. The metacmle is a shallow space on the dorsal aspect of the 

 medulla oblongata, overlapped in front for a short distance by the 

 cerebellum, and behind covered only by the pia mater, containing 

 a network of vessels, the choroid plexus of the metaccele (Fig. 971, 

 pch). At the point where medulla oblongata and spinal cord meet 

 is a strong ventral flexure. 



The spinal cord is continued backwards throughout the length 

 of the neural canal, becoming slightly dilated opposite the origins 

 of the two pairs of limbs and tapering greatly towards the 

 posterior end of the tail. 



The cerebral nerves resemble those of the Frog as regards their 

 origin and distribution in most respects, the principal difference 

 being that a spinal accessory is 

 intercalated in front of the hypo- 

 glossal, and that the hypoglossal 

 arises from the medulla oblongata, 

 not from the spinal cord, and is 

 therefore a cerebral nerve. 



The nasal cavities (Fig. 972) open 

 at the extremity of the snout by 

 the external nares, and into the 

 cavity of the mouth by a pair of 

 slit-like internal nares situated near 

 the middle line of the palate. The 

 external aperture opens into a sort 

 of vestibule, beyond which is the 

 nasal or olfactory cavity proper, 

 containing a convoluted turhinal lone over which the mucous 

 membrane extends. Opening into each nasal cavity, near the 

 internal opening, is Jacohson's organ {J. J.), an oval sac with strongly 

 pigmented walls supported by cartilage. 



The eye has a cartilaginous sclerotic having a ring of small bones 

 (Fig. 973) supporting it externally. There is a cushion-like jsectei or 

 vascular pigmented process similar to the struc- 

 ture of the same name occurring in Birds (see 

 below, Class Aves), projecting into the inner 

 chamber of the eye. In essential structure the 

 rest of the eye agrees with that of the Craniata 

 generally as already described. Two glands 

 lie in the orbit, the lacrymal and the Har- 

 derian. 



The ear consists of two principal parts, the 

 middle ear or tympanum, and the internal ear or membranous laby- 

 rinth. The former is closed externally by the tympanic membrane, 



Fig. 972. — Transverse section of the nasal 

 region of the head of Ziacerta to 

 show the relations of Jacobson's or- 

 gans. D, nasal glands ; /. /. Jacob- 

 son's organs ; N, N. nasal cavities. 

 (From Wiedersheim's Comparative 

 Anatomy.) 



Pig. 973.— Ring of ossicles 

 in sclerotic of eye 

 of Iiacerta. (After 

 Wiedersheim.) 



