lo Dec, ipoS.] Elements of Animal Physiology. 719 



happens that in this case the region of the grey matter where the distri- 

 bution is effected is small and circumscribed and we thus speak of the 

 centre for defsecation. A trifling injury to the cord may destroy this 

 centre and rob the animal of its power to defaecate normally. Similarly 

 there are centres for the bladder and for the sexual organs. It must be 

 noted in this connexion that, when muscles are involved in a reflex, it is not 

 enough merely for certain muscles to be activated — others must be stopped 

 or inhibited. Thus in the act of defaecation the sphincter of the anus 

 must relax : when in the act of scratching, the hock is -flexed it will be 

 found that the muscles which extend the hock lose their tautness or 

 " tone " and vice versa. The muscular and visceral reflexes carried out 

 by the spinal cord, as also those of the brain stem, have a distinct pur- 

 posive character ; thus a spot tickled is scratched, faeces and urine are 

 expelled, a limb is withdrawn from a source of injury, &c. In the whole 

 central nervous system, except the fore-brain, we find that the chains of 

 neurons along which the impulses pass are definite and inborn. Certain 

 stimuli or sets of stimuli will provoke certain responses and only these. 

 It will be apparent, from what has been stated above, that a reflex may 

 be lost through injury to receptors, or to afferent fibres, or to the central 

 nervous system, or to exit fibres, or to the receptive substance or active 

 structure of the muscle or gland. 



The mxcdulla oblongata is really a continuation of the spinal cord 

 headwards. The typical arrangement of the grev and white matter is 

 however disturbed, the central core of grey matter being broken up bv 

 strands of white fibres, whilst special accumulations of nerve cells occur 

 in the midst of the white matter. It is in this region that the great strand 

 of white fibres, on either side, passing from the forebrain to the spinal 

 cord, crosses from one side to the other and produces what is called the 

 ' decussation of the pyramids." It is owing to this decussation that an 

 apoplectic seizure involving, say, the right side of the forebrain, con- 

 ditions a partial paralysis of the left side of the bodv. 



In the PONS the distribution of grey and white matter is still further 

 disturbed by the presence of fibres entering and leaving the cerebellum 

 and connecting together the two lateral expansions of this organ. Taking 

 the TOns and the medulla together it may be briefly stated that the nerves 

 arising from this region are the 5th cranial or trigeminal, the 6th cranial 

 cr abditcens, the 7th cranial or facial, the 8th cranial or auditory, con- 

 taining however fibres from the semi-circular canals as well as fibres from 

 the cochlea, the 9th cranial or glossopharyngeal, the loth cranial or vagus, 

 the nth cranial or spinal accessory, and the 12th cranial or hypoglossal. 

 The afferent supply of the medulla-pons region is large and embraces 

 the following — from the skin of the forehead and face, the teeth, and 

 the mucous membranes of the mouth, eye and nose — the.se enter by the 5th ; 

 from the organ of hearing and from that great proprioceptive organ, the semi- 

 circular canal .system — these enter by the 8th ; from the mucous membranes 

 of the pharynx, back of the tongue and the Eustachian tube, and probably 

 from the taste receptors — these enter by the 9th ; from the heart and aorta, 

 the trachea, bronchi and lung, and the mucous membranes of the alimen- 

 tarv canal from the oesophagus to the colon — ^these enter by the loth ; 

 .^nd finally from the muscles innervated by the motor outflow from this 

 region. As the medulla and pons are not i-solated from the rest of the 

 C.X.S we accordinglv find that the afferents of the spinal cord send up 

 branch fibres that enter here into the grey matter, and similarly afferents 



