522 A MANUAL OF VETERINARY PHYSIOLOGY 



being stretched, and so rendered the animal mute. If the 

 peripheral end of a divided recurrent be stimulated, spasm 

 of the larynx is produced. There are certain vegetable poisons, 

 such as those contained in Lathyrus sativus, and others of the 

 Leguminosae, which appear to especially single out this nerve. 

 Horses fed on grain containing Lathyrus are soon rendered 

 incapable of work, as asphyxia occurs through spasm of the 

 larynx. 



Chauveau, in his experimental inquiry on the vagus of the 

 horse, found that if both vagi be divided in the neck and the 

 animal fed, the stomach and whole length of the oesophagus 

 frequently became greatly distended with food, in consequence 

 of loss of motor power. It seems certain that the sensation of 

 repletion in these cases is lost, which would account for the 

 animal continuing to eat. 



Modern Classification of Cranial Nerves. 



The modern conception of the architecture of the cranial nerves 

 is to regard them as built on the same lines as spinal nerves. In 

 this scheme the first and second pairs are not included, the olfactory 

 bulb and retina being regarded as outgrowths from the brain. From 

 the third pair to the twelfth the cranial nerves originate from the 

 brain stem — i.e., the bulb, pons, and mid-brain — which is only an 

 extension forward of the spinal cord. Scattered irregularly in this 

 position, but mainly in the region of the fourth ventricle, are groups 

 of nerve-cells, or nuclei, some of which are sensory, others motor. 

 The motor nuclei are arranged in two longitudinal rows, on either side 

 of the primitive neural axis. From the median row arise the third, 

 fourth, sixth, and twelfth nerves ; from the lateral row the motor 

 branches of the fifth, seventh, ninth, tenth, and eleventh. 



Nuclei which have been termed sensory are found in the same 

 region connected with the fifth, eighth, ninth, and tenth nerves. 

 These nuclei are the terminations of the sensory cranial nerves, for 

 the latter are arranged in the same way as a spinal nerve, and have a 

 ganglion on their sensory root. The cranial nerves with their ganglia 

 are outside the cranial cavity. The ganglia consist of cells, each, 

 like a spinal nerve ganglion, possessing a T-shaped process, one end 

 of which grows to the centre and one to the periphery. It is the 

 central end of the T-piece which terminates in the sensory nucleus of 

 the brain stem, by arborising around its cells. It will be observed 

 that the sensory fibres do not arise in the nuclei, but terminate 

 there. The resemblance between the cranial and spinal nerves is 

 completed by the existence of sympathetic fibres in the cranial 

 system. 



Under this scheme the sensory portion of the fifth pair represents 

 the dorsal roots of the following* motor (ventral) cranial nerves — i.e., 

 seventh and twelfth. The tenth pair becomes the dorsal root of the 

 eleventh. The ninth becomes a dorsal, with no ventral root. The 

 eighth nerve has its dorsal root from the cochlea, and its ventral 

 from the vestibule and semicircular canals. 



