472 A MANUAL OF VETERINARY PHYSIOLOGY 



mediate set of neurones, connecting the cell-termination of the 

 sensory nerve with the cell origin of the motor. 



|Special Centres in the Spinal Cord.— In the cord certain centres 

 exist, which, though ordinarily under the control of a chief 

 centre in the bulb, yet are capable, as we have seen, of carrying 

 on peculiar reflex actions even after the cord has been divided. 



The cilio-spinal centre lies between the cervical and dorsal 

 portions of the cord ; in it fibres originate which through the 

 cervical sympathetic supply the dilator muscle of the iris. 

 Destruction of the region in question causes a contraction of the 

 pupil, whilst irritation of it causes the pupil to dilate. The 

 ano-spinal centre, found in the lumbar portion of the cord, controls 

 the act of defalcation ; it would appear to be highly developed in 

 herbivora, which possess the power of bringing it into play not 

 only when the body is at rest, but during movement. The 

 functions of the ano-spinal centre appear rather complex, 

 inasmuch as it has not only to maintain the tone of the sphincter, 

 but also to relax it during defalcation, and under the latter 

 condition simultaneously to contract the abdominal muscles 

 and diaphragm. These, however, are brought about quite 

 simply through the reciprocal innervation of antagonistic 

 muscles (see p. 456). The vesicospinal centre also exists in the 

 lumbar portion of the cord, and governs micturition ; its action 

 is similar to that of the ano-spinal centre. In the lumbar 

 portion of the cord other centres are found — for example, the 

 erection centre, the genito-spinal centre which contains the 

 nervous apparatus employed in the emission of semen, and the 

 parturition centre. Vasomotor centres are found throughout the 

 cord ; they are principally under the control of similar centres 

 in the bulb, but may act independently. All these centres in 

 the cord are activated through the sympathetic system. 



