352 PHYSIOLOGY CHAP. 



their axis-cylinder. Lapinsky's results contradicted the conclusions 

 of previous workers that the motor centres of the cord are segment- 

 ally arranged in correspondence with the respective segments of the 

 limbs they innervate. The cord has no compact columns of cells, 

 but merely solitary groups at different levels with nO definite 

 boundaries. Still less is it possible, he says, to demonstrate special 

 centres for the flexors and extensors or for the adductors of the 

 thigh. The cells with these functions lie at different levels of the 

 cord and belong to the groups which simultaneously supply their 

 antagonist muscles. So, too, the idea that each muscle has its 

 special centre is contradicted by the fact that every muscle receives 

 nerve -fibres from several ventral roots, and that each of the 

 larger muscles has centres in several different groups of cells. No 

 experiments have yet succeeded in demonstrating distinct centres 

 in the cord for separate muscles, or groups of muscles with the 

 same function. 



Owing particularly to Goltz, who made a prolonged study of 

 the effects of complete transection of the cord in the lower thoracic 

 region, we are able to divide the spinal reflex centres into two 

 groups: those seated in the lumbo-sacral part of the cord and 

 those in the cervico-thoracic part. The two enlargements, lumbar 

 and cervical, may physiologically be regarded as two lower or 

 subordinate brains, which preside over the sum of the reflex acts 

 of which these two parts of the cord are capable. 



The lower or lumbo-sacral part of the cord contains the centres 

 for the following special reflexes : 



(a) The centre for movements of the posterior (lower) limbs ; 

 as we have seen, it is possible in the " spinal " dog with suitable 

 stimulation of the skin to evoke all the reflex acts of which 

 the lower part of the animal's trunk is capable. 



(6) The ano-spinal centre (Vol. II. p. 372). 



(c) The vesico-spinal centre (Vol. II. p. 474). 



(d) The centre for erection, the genito-spinal centre, and the 

 ecbolic or utero-vaginal centre (which we shall discuss in the 

 chapter dealing with the functions of the male and female genital 

 systems ; see Vol. V.). 



The upper or cervico-thoracic region of the cord contains : 



(a) The centre for movements of the anterior (upper) limbs. 



(6) The spinal centres for the respiratory movements (Vol. I. 

 p. 447). 



(c) The spinal centres for the cervical sympathetic, the vaso- 

 motor and secretory fibres of which run principally to the head. 

 The so-called cilio-spinal centre, or dilatator of the pupil, dis- 

 covered by Budge, extends from the lower half of the cervical cord 

 to the level of the third thoracic segment. Electrical excitation 

 of this segment region produces mydriasis, like the excitation of 

 the cervical sympathetic. 



