THE CAT 247 



a large, irregularly shaped head. The handle extends across the 

 dorsal portion of the tympanic membrane, above the hinder rim 

 of which the head is situated. The anvil is a minute bone with 

 two projections; it articulates with the head of the hammer by 

 one of its limbs and with the stirrup by the other. The last- 

 named bone, which has the form of a stirrup, fits into the fenestra 

 ovalis, an opening into the inner ear ; this opening is just medial 

 to the ridge surrounding the fenestra rotunda. 



Exercise 37. Draw a diagrammatic sketch of the middle ear, so far 

 as observed. 



The Inner Ear. The membranous labyrinth is an irregular sac 

 entirely inclosed in the bony capsule called the bony labyrinth. 

 It communicates with the middle ear by two openings,— the fe- 

 nestra ovalis and the fenestra rotunda, just mentioned,— and con- 

 sists of three main portions,— the vestibule, the cochlea, and the 

 semicircular canals. The cochlea is situated in the prominent ridge 

 which appears just back of the fenestra ovalis. The vestibule is 

 behind the cochlea, which extends from it on one side ; from the 

 opposite side project the semicircular canals. 



The bone within which the membranous labyrinth lies is very 

 hard and thick, and a dissection of the labyrinth is impossible 

 without a special treatment of the bone. If, however, this portion 

 of the skull is decalcified in a 10 per cent solution of nitric acid, 

 the labyrinth can be studied. 



The Central Nervous System; the Brain. Before removing the 

 brain from the cranial cavity cut away the skin and the muscles 

 from the head and the adjacent portions of the neck ; remove also 

 the tongue and the lower jaw. With bone forceps chip away the 

 roof of the skull. Do not tear, if avoidable, the dura mater, the 

 outermost of the coatings of thel>rain, which lies next to the in- 

 ner surface of the skull, but press it away from the skull as the 

 removal of the bone continues. Expose thus the dorsal and lateral 

 surfaces of the brain. Take pains to preserve intact the olfactory 

 lobes, which form the anterior end of the brain. Special care must 

 also be taken in removing the tentorium, the vertical bony parti- 

 tion which extends from the roof of the skull between the cere- 



