618 PHYSIOLOGY OF THE BRAIN-STEM 



6. FUNCTIONS OF THE BRAIN-STEM AS A WHOLE 



While it is not possible as yet to name the exact functions of the separate 

 centers in the corpora quadrigemina, the optic thalami and certain other 

 parts of the brain-stem,, we have some observations on decerebrated animals 

 which should afford us some light as to the functions of the hindbrain, 'tween- 

 brain and midbrain taken together. What the central nervous system is 

 capable of without the cerebrum., considered in connection with the functions 

 remaining after removal of all the parts anterior to the medulla,, should give 

 us a general idea of the total powers of the brain-stem. 



The lowest vertebrate, Amphioxus, has no true encephalon: its brain consists 

 only of a slight enlargement at the anterior end of the spinal cord. Anteriorly 

 and laterally this enlargement embraces a ventricle which is continuous poste- 

 riorly with the central canal of the spinal cord. This " brain " contains internally 

 a ganglionic mass and externally a mass of nerve fibers. The former consists 

 mainly of multipolar cells, whose fibers pass over into the fibers of the outer layer 

 running longitudinally of the animal. 



Steiner divided this " fish " into two pieces, a head and a tail piece. After 

 some minutes both parts responded to mechanical stimulation by making per- 

 fectly regular locomotor movements, at the same time preserving equilibrium 



and swimming with the anterior end of the 

 piece forward. They fell over on their broad 

 side however as soon as the movement ceased. 

 Olfactory nerve The animals could even be cut into three - or 



four pieces, and under the circumstances named 

 (hemispheres) each part could still make locomotor move- 



Midbrain ments. Steiner's conclusion is that the body 



Hindbrain & ^ Amphioxus consists of perfectly equivalent 



(cerebellum) metameres and has in general no motor center. 

 ' A (medI35a Danilewsky obtained results of very differ- 



oblongata) ent purport. After division of the animal into 



an anterior and a posterior half, he noticed in 

 the piece containing the brain occasional " vol- 

 FIG. 274. The brain of Sgualius untary " movements apparently independent of 

 cephalus, a bony fish, after Steiner. any externa l stimulus; while the posterior half 



remained perfectly motionless. By artificial 



stimulation movements could be obtained in the anterior half more easily than 

 in the posterior. They continued for from fifteen to thirty seconds after stimu- 

 lation and consisted of a series of bending and stretching movements. 



When the head was cut off, the above-mentioned voluntary movements ceased. 

 The animal then lay for one to two days without spontaneously changing its 

 position in any wise, when care was taken to remove all external stimuli. The 

 reflex movements to artificial stimuli were perfectly normal, but were not abun- 

 dant, and the irritability of the headless animal was considerably less than that 

 of the isolated anterior half. 



Danilewsky concludes from these and other observations that the so-called 

 brain of the Amphioxus contains the centers for voluntary motions; destruction 

 or separation of these from the rest of the cerebral nervous system results in 

 loss of motility, providing no external stimulus of sufficient strength acts upon 

 the animal. 



