,76 ZOOTOMY. 



and mandibular. teeth) : there is also a small patch of tee^^ 

 on each side of the roof of the mouth (palatine teeth). = 



219. The posterior nares, considerable apertures 

 situated a Httle behind the end of the snout and 

 separated from one another by a vertical partition 

 supported by the vomers (§ 63). 



Slightly in front of the posterior nares are two minute slit-like aper- 

 tures, each leading into a small pigmented sac (Jacobson's organ) 

 lying under the nasal sac. The relations of these parts are best seen in 

 a transverse section of the snout. 



220. The Eustachian pits ; large paired diverticula of 

 the dorsal wall of the pharynx, continuous with the tym- 

 panic cavities, which are bounded externally by the 

 tympanic membranes. 



221. The position of the columella auris in the tympanic cavity, 

 and its relations to the fenestra ovalis and tympanic membrane, may 

 now be made out (see § 65, p. 142). Also remove an eye and note a 

 series of thin scale-lil<e bones (sclerotic plates) surrounding the border 

 of the sclerotic. 



XXVI. Break away the roof of the skull with bone 

 forceps so as to expose the brain : this may be 

 done without first dissecting off the skin which 

 adheres very closely to the subjacent bones. Re- 

 move the brain, taking especial care not to injure 

 the olfactory lobes (§227, Fig. 45, olf), and, after 

 preservation in spirit, place it under water in a 

 small dissecting dish, and make out its parts as 

 follows : — 

 222. The medulla oblongata (Fig. 45, m.d), or pos- 

 terior division of the brain, continued insensibly behind 

 into the spinal cord : it is about double the diameter of 

 the cord, and exhibits a strong ventral flexure. 



