816 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



In the case of mammals it ought, perhaps with more correctness, 

 to be stated that these highly co-ordinate movements owe their perform- 

 ance to parts above the medulla, since the removal of the optic thalami, 

 crura cerebri, corpora quaclrigemina, cerebrum, and pons varolii is 

 usually followed by various forced movements, or the animal lies partly 

 on its side, and, although various complex movements may be produced, 

 they are much more limited than when the higher parts of the brain are 

 left intact. 



The medulla oblongata is the seat of a large number of centres 

 governing complex co-ordinate movements, of which the following are 

 the most important : — 



(a) The Respiratory Centre. — This is located in the medulla behind 

 the point of origin of the pneumogastric nerves on both sides of the 

 apex of the calamus scriptorius, between the nuclei of the vagus and 

 spinal accessory nerves. This centre is symmetrically situated, and 

 may be separated by a longitudinal incision without interfering with 

 the respiratory movements. Conditions governing the actions of this 

 centre have been already given under the subject of respiration. 



(b) The Cardio-Inhibitory Centre. — As has been previously stated, 

 when the pneumogastric nerve is stimulated it may slow or arrest the 

 heart in diastole, according to the degree of stimulation. The inhibitory 

 fibres of the pneumogastric reach the latter nerve through the spinal 

 accessory and have their origin in the medulla oblongata. The conditions 

 for its action have been likewise given. 



(c) The Vaso-Motor Centre. — The collection of nerve-cells which 

 govern the vaso-motor nerves, and through them the calibre of the blood- 

 vessels, is located in the floor of the medulla oblongata, extending from 

 the upper part of the floor to within four or five millimeters of the col- 

 umns of the cerebellum. This centre is also a double one, situated sym- 

 metrically on each half of the medulla and each part corresponding to the 

 upward continuation of the lateral columns of the spinal cord. Stimula- 

 tion of this centre causes contraction of all the arteries with a consequent 

 increase in the blood pressure ; paralysis causes relaxation and dilatation 

 of the blood-vessels and fall of blood pressure. Its action has also been 

 described. 



(d) Centre for Closure of the Eyelids.— The centre for the closure 

 of the eyelids lies close to the calamus scriptorius. The afferent impulses 

 reach it through the sensory branches of the fifth cranial nerve from 

 stimuli applied to the cornea, conjunctiva, and skin in the neighborhood 

 of the eye ; reaching the centre of impulse, are transferred to the special 

 nerve, through which the afferent impulses reach the orbicularis palpe- 

 brarum. 



(e) Centre for Sneezing.— The location of this centre has not been 



