S14 THE APPARATUS OF THE SENSES. 



The tongue and its investing membrane having been described at page 

 335, their anatomy need not again be referred to ; but we must glance at 

 the organisation of the latter in considering it as the special apparatus of 

 gustation. This will necessitate a few words on the free surface of the mem- 

 brane which comes into contact with the sapid bodies, and some consider- 

 ations on the terminations of the nerves which transmit the impressions 

 produced by these bodies to the brtdn. 



FREE SUEFACE OP THE LINGUAL Mucous MEMBRANE. This surface is 

 studded by a multitude of papillary prolongations, which are nearly all 

 limited to the upper surface of the tongue, to which they give a tufty 

 appearance. Their form and volume, as mentioned at page 336, are very 

 variable, according to their situation : some are microscopic, while others 

 form voluminous caruncles ; others, again, are long, conical, and filiform ; 

 another variety is round or depressed, representing a hemispherical tubercle 

 scarcely projected beyond the general surface, or placed at the bottom of an 

 excavation in the mucous membrane. The latter constitute the calyciform 

 papillce (p. circumvallatce, p. lenticulares), and are considered the true organs 

 of gustation ; the others are the fungiform (p. capitatce) and filiform papillce, 

 which play a mechanical part on the surface of the tongue. 



The calyciform papillce in the Horse are two in number, and situated near 

 the base of the tongue ; their diameter is so considerable that they have been 

 named the blind or caecal openings (trous borgnes). They are the principal, 

 but not the only organs of taste. Their surface is mammillated, each 

 prominence corresponding to a single papilla, and being placed below the 

 level of the raised border encircling them. A deep fossa surrounds them, 

 and limits at their base a pedicle, which unites them to the other portions 

 of the mucous membrane. 



The calyciform papillae show, around their peduncle, a band of adenoid 

 tissue ; and in their substance conglomerate glands, as in other parts of the 

 mucous membrane. They are covered by an epithelium containing some 

 scattered pigment granules, the thickness of which is much diminished at 

 the bottom of the fossa circumscribing them. 



TERMINATION OF THE GUSTATORY NERVES. The hypoglossal is the motor 

 nerve of the tongue, the lingual the nerve of general sensibility, and the 

 chorda tympani and glosso-pharyngeal the filaments of special sensibility : 

 this appears to be clearly ascertained from the recent experiments and 

 observations of Lussana. The lingual branch of the glosso-pharyngeal 

 nerve gives gustatory sensibility to the posterior third of the tongue ; the 

 chorda tympani to the anterior two-thirds. 



The gustatory nerves present, as do all those of the organs of sense, a 

 particular mode of termination. First indicated by Axel Key, their special 

 manner of terminating has been carefully studied by Lowen and Schwalbe. 

 According to these anatomists, the terminal nerve-tubes lose their medul- 

 lary envelope and, reduced to their axis-cylinder, are thrown out in small 

 oval masses which might be termed gustative bulbs. These bulbs are more 

 particularly placed around the pedicles of the calyciform papillae, in the 

 substance of the epithelium. They are fusiform, their inner extremity 

 rests on the mucous derma, where they receive the terminal nerve-tubes; 

 and their external extremity reaches the epithelial layer, where they are seen 

 either between two cells, or in an orifice pierced in a single pavement cell. 

 Each bulb is composed of a small cluster of cells, which are distinguished 

 from each other by their character and position ; those occupying the axis 

 of the organule are the gustative cells; they are in communication with the 



