682 PHYSIOLOGY. 



duces hemiplegia, but only paresis. Equally, in animals this same 

 lesion does not entirely abolish sensibility in the opposite side of the 

 body. The gray substance of the opposite side connects the parts 

 lying over and under the lesion, and so conducts the sensory im- 

 pressions. 



Bulbar Nerves. From the medulla oblongata many pairs of 

 nerves, the bulbar nerves,, take their origin and departure. Each nerve 

 has a gray nucleus. The nuclei on the right side are connected with 

 those on the left and all have their location along the gray substance 

 of the floor of the fourth ventricle. The fibers which connect these 

 nuclei of origin with the superior cranial centers are also crossed on 

 the way. 



Centers. The medulla, with its gray substance and especially 

 with the gray nuclei of the nerves which issue from it, becomes a 

 center of very important functions. 



First, it is a respiratory center. This center is found toward the 

 inferior angle of the fourth ventricle, a little back of and lateral to 

 the source of the vagi nerves. It is composed of two lateral halves, 

 each of which, in function, can take the place of the other. This 

 center is about two and one-half millimeters in size. 



A lesion affecting loth respiratory centers causes the sudden 

 death of a warm-blooded animal. Therefore, this region of the 

 fourth ventricle has been called the vital knot. In fact, a blow from 

 a stick upon the back part of the head or upon the nape of the neck, 

 also a thrust from a sharp stilleto between the back of the head and 

 the first vertebra, suffices to cause even a large mammal to fall to the 

 ground instantly. Butchers do this because they injure the vital 

 knot. 



COMPONENTS OF THE CENTER. The center of respiration in the 

 medulla is composed of an inspiratory center and an expiratory center. 



From the inspiratory center the excitation for the nerves, and 

 therefore for the muscles of inspiration, takes its departure rhyth- 

 mically. These motor excitations always decussate in the cervical 

 cord. The inspiratory excitation reaches the center by means of the 

 pneumo gastric nerves, having been carried along their sensory pulmon- 

 ary fibers. The excitation is originated either by reason of an accumu- 

 lation of C0 2 in the blood or the absence of 0. On the contrary, 

 an excess of oxygen in the blood abolishes excitation of the inspira- 

 tory center. 



The expiratory center, on the other hand, gives excitation to the 



