CENTRES IN THE PONS. 891 



For further description of the functions of the region, it is convenient 

 to follow the morphological subdivision of it into a floor and roof, the 

 former the pons of higher vertebrates, the latter the cerebellum. 



THE PONS. 



All the root cell system of the metencephalon belongs to its floor. 

 The afferent root cells include the collection of the trigeminal root 

 conducting impulses from the cutaneous sense organs of the forehead 

 and face and from the mucous membranes of the nose, eye, and mouth. 

 It is not clear whether the afferent root cell system in the metencephalon 

 includes any units from sense organs of muscle. If the eighth nerve 

 be reckoned to this region, the system includes the sense organs for the 

 spatial sense of the head, and in air-breathers the sense organ for 

 hearing, which in mammals comes to rival in importance that of vision 

 itself. 



The efferent root cell system of the region discharges into muscles 

 that execute prehension of food and mastication, facial expression 

 (emotion), and that oral modification of throat sounds that contributes 

 so largely to vocal expression and to speech. It is probable that the 

 frontalis and orbicularis palpebrarum muscles get their motor inner- 

 vation only through and not originally in this region. Lateral movement 

 is also in part furnished to the retina from this region. Some of the 

 fibres in the facialis root come from cells of the crossed facialis 

 nucleus. 1 



For the details of the special physiology of these nerves, reference 

 can be made to the appropriate sections of this work. Out of those 

 details features which rise as salient characteristics of the function of 

 the region may be briefly stated thus. Into it extends the cephalic end 

 of the great neural system for the reception of impulses generated in 

 sense organs of the skin and its appendages, and since much of this 

 region lies inaccessible to the visual field, their importance is in so 

 far unique. The cephalic end of the skin connotes " most headward 

 pain." Tactual space perception of the head directs prehension of 

 food, though least in man. Among its sense organs are those contri- 

 butory to the testing of objects in regard to their suitability for food, 

 and alongside these lies a main part of the system for discharge 

 into the muscular organs for the prehension and buccal examina- 

 tion and preparation of food. In this region enter the impulses from 

 the organ that, comparably with the visual, forms the basis of an 

 enormous superstructure of highest nervous reaction, and a medium for 

 communication of feelings and ideas. Thirdly, into it pour the vesti- 

 bular impulses informing as to position of the head, and so in great 

 measure as to position of the body as a whole, and thus regulating 

 the activity of great fields of skeletal musculature. 



A number of "centres" are described in this metencephalic floor, 

 many of which can be inferred directly from its efferent root cell 

 groups. Among the reflexes localised within it may be mentioned the 

 following : Closure of the eyelids on touching the conjunctiva, in man 



1 Lugaro, Untersuch. z. Naturl. d. Mensch. u. d. Thiere, Bd. xv. ; Cajal, "Apuntes 

 para el estudio del bulbo, etc.," Madrid, 1895; van Gehuoliten, " Le system e nerveux," 

 2nd edition, Gand, 1896; v. Pngliese and v. Milla, Riv. sper. difrenmt., Reggio-Emilia, 

 1896, torao xxii. p. 805 ; Dejerine and Theobari, Compt. rend. Soc. de MoL, Paris, 4th Dec. 

 1897 ; G. Marinesco, Rev. neurol., Paris, 1898, tome iv. p. 30. 



