. FUNCTIONS OF THE MEDULLA OBLONGATA, 659 



go through it), and then passes downwards, forwards, and outwards, to a column of cells more 

 deeply placed in the medulla than any other nucleus in the lower part." But the 7th has no 

 real origin from this nucleus. Facial Nerve. The nucleus lies deep in the formatio reticularis 

 of the pons under the floor of the 4th ventricle, but outside the position of the nucleus of the 

 6th (fig. 427, 7). It extends downwards about as far as the auditory striae, or a little lower. 

 The fifth nerve arises from its motor nucleus (with large multipolar cells), which lies more 

 superficially above and external to the 6th (fig. 427, 5), The fibres run backwards, where they 

 are joined by fibres from the upper sensory nucleus, but another sensory nucleus extends down 

 nearly to the lower end of the medulla (5"). Doubtless this extensive origin brings this nerve 

 into intimate relation with the other cranial nerves, and accounts for the numerous reflex acts 

 which can be discharged through the fifth nerve. Some sensory fibres are said to pass up 

 beneath the corpora quadrigemina (Goooers). The fourth nerve arises from the valve of 

 Vieussens, i.e., the lamina of white and grey mutter which stretches between the superior 

 cerebellar peduncles. It arises, therefore, behind the 4th ventricle, but some of the fibres spring 

 from nerve-cells at the lower part of the nucleus of the 3rd nerve. Some fibres also descend in 

 the pons to form a connection with the nucleus of the 6th nerve. The fibres decussate behind 

 the aqueduct, so that in it alone, of all the cranial nerves, decussation occurs between its 

 nucleus and its superficial origin {Growers). The third nerve arises from a tract of cells beneath 

 the aqueduct and near the middle line, and the fibres descend through the tegmentum to appear 

 at the inner side of the crus cerebri. Gowers points out that, in reality, there are three distinct 

 functional centres, (1) for accommodation (ciliary muscle), (2) for the light reflex of the iris, 

 and (3) most of the external muscles of the eyeball. It is important to notice the connection 

 between the nuclei of the 3rd, 4th, and 6th nerves, in relation to the innervation of the ocular 

 muscles. ] 



Functions. The medulla oblongata, which connects the spinal cord with the 

 brain, has many points of resemblance with the former. [Like the cord it is 

 concerned (1) in the , conduction of impulses.] (2) In it, numerous reflex 

 centres are present, e.g., for simple reflexes similar to the nerve-centres in the spinal 

 cord, e.g., closure of the eyelids, [so that they subserve the transference of afferent 

 into efferent impulses]. There are other centres present which seem to dominate 

 or control similar centres placed in the cord, e.g., the great vaso-motor centre, the 

 sweat-secreting, pupil-dilating centres, and the centre for combining the reflex move- 

 ments of the body. Some of the centres are capable of being excited reflexly 

 ( 358, 2). (3) It is also said to contain automatic centres ( 358, 3). The 

 normal functions of the centres depend upon the exchanges of blood-gases, effected 

 by the circulation of the blood through the medulla. If this gaseous exchange be 

 interrupted or interfered with, as by asphyxia, sudden anaemia, or venous congestion, 

 these centres are first excited, and exhibit a condition of increased excitability, and 

 at last, if they are over-stimulated, they are paralysed. An excessive temperature 

 also acts as a stimulus. All the centres, however, are not active at the same time, 

 and they do not all exhibit the same degree of excitability. Normally, the 

 respiratory centre and the vaso-motor centre are continually in a state of rhythmical 

 activity. In some animals, the inhibitory centre of the heart remains continually 

 non-excited; in others, it is stimulated very slightly under normal conditions, 

 simultaneously with the stimulation of the respiratory centre, and only during 

 inspiration. The spasm centre is not stimulated under normal conditions ; and 

 during intra-uterine life, the respiratory centre remains quiescent. The medulla 

 oblongata, therefore, contains a collocation of nerve-centres which are essential for 

 the maintenance of life, as well as various conducting paths of the utmost 

 importance. We shall treat of the reflex, and afterwards of the automatic centres. 



367. REFLEX CENTRES OF THE MEDULLA OBLONGATA. The 



medulla oblongata contains a number of reflex centres, which minister to the 

 discharge of a large number of co-ordinated movements. 



1. Centre for closure of the eyelids. The sensory branches of the 5th cranial 

 nerve to the cornea, conjunctiva, and the skin in the region of the eye, are the 

 afferent nerves. They conduct impulses to the medulla oblongata, where they 

 are transferred to, and excite part of, the centre of the facial nerve, whence, through 



