Page:The New International Encyclopædia 1st ed. v. 14.djvu/434

* NERVOUS SYSTEM AND BRAIN. 384 NERVOUS SYSTEM AND BRAIN. cord along the aiitciior median fissure. The posterior spinal arteries also arise from the vertebral arteries and pass down the posterior aspect of the cord aloii'^ the line of the posterior nerve roots. Branches from the spinal arteries form an extensive anastomosis in the pia mater. From this network brandies pass into the sub- stance of the cord. These arteries, like those in the brain, do not anastomose after entering the nervous tissue proper. FuxcTio.NS OF THE Cerebeum. The brain, and more especially the cerebral hemispheres, must lie consi<lered as the organ of the mind. It is liere that those material activities take place vhich underlie those mental attributes which we recognize as consciousness, intellect, emotions, and will. In the case of the higher animals removal of the cerebrum results so promptly in death of the animal that few conclusions can be drawn. In one of the lower animals, e.g. the frog, the result is the destruction of all volition and apparently of all consciousness. Stimula- tion, such as pincliing the toes or applying the electric current to nerves or muscles, causes muscular contraction and consccpu'nt movement, but if left entirely alone the frog shows no sign of movement, with the exception of the automatic movements connected with the heart beat and with breathing. In some cases where after a time slight voluntary movements seemed to occur, it was found that small jiortions of the brain liad not been removed. Professor Dalton's de- scription of a pigeon from which the cerebrum liad been removed is as follows: "The elFcct of this mutilation is simply to plunge the animal into a state of jirofound stupor in which it is almost entirely inattentive to surrounding ob- jects. The bird renuiins sitting motionless upon his perch or standing upon the ground with eyes closed and head sunk between the shoulders. The plumage is smooth and glossy, but is luii- formly expanded by a kind of erection of the feathers, so that the body appears somewhat puffed out and larger than natural. Occa^iiinally the bird opens its eyes with a vacant stare, stretches its neck, perhaps shakes its bill once or twice, or smooths down the feathers upon its shoulders, and then relapses into its former apathetic condition." The brain would thus seem to be essential to any conscious activity. Whatever functions are carrie'd on by the rest of the nervous system enter into consciousness only through the brain. Thus stimulation ot a sensory nerve may be concciveil as causing .a motion (as in the case of the frog without a cerebrum) entirely independent of the brain, but in such a case there is neither any consciousness of the stimulation (sensation), nor any knowl- edge or control of the resulting motion. It is a purely retlex net. By menus of the brain, then, stinuilation of sensory nerves results in an ac- tivity in consciousness which we call sensation, and by the same nu-ans both knowledge of and control over motion are accom)dislied. The so- called higher mental activities, intellect or thought, and feeling or emotion, are also entirely dependent upon the cerebrum. Our knowledge of the localization in the brain of its various functions is still far from perfect. Much of the experimentation upon cerebral localization hns been done upon the nuinkey by exposing the brain and electrically stinuilating its surface. The work of Fcrrier along this line has been of exlreiue importance. By electrically stimulating the cortex in the monkey he deter- mined the area stimulation of which caused mo- tion of some part of the body, and, by eiimi)aring the convolutions of the monkey's brain with those of the human brain, he determined the probably analogous areas. The results may be brielly summed uj) as follows: There is a eonsidcralde area having the tissure of Kolando for its central line, which is known as the general sensory- motor area. The motor relations of this area are well shown in the accompanying diagrams. Stim- ulation of other parts of the brain failed to elicil any reaction. The sensory areas are not so well ilclined. and for obvious reasons are much more dillicult of determination. Within this same THE LKFT C£REDR.Li BEMISPHEKE (HUMAN) SHOWI.NO THE DIFFEHENT LOCALIZAKLE AKEAS OS THE EXTERNAL SURFACE. region are ])robably located the areas which have to do with general sensations. The special senses have, however, si)ecial localizations. Thus the cortical visual centre is sittiated in the occi- pital lobe, the centre for the conscious apprecia- tion of sound (hearing centre) in the fnmt and upper part of the temporal lobe, while the sense of smell is located below the latter in the tip of the temporal lobe. That the frontal lobes are most concerned in the higher intellectual activities has long been believed. Stimulation of these lobes causes no reaction. According to Ferrier their complete removal causes no sensory or motor disturl>ances. His description of the nu)nkey after complete re- moval of the frontal lobes is as follows: "The animals retain tlicir appetites and instincts and are capable of exhibiting emotional feeling. The sensory faculties — sight, hearing, touch, taste, and smell — remain imimpaired. The (lowers of voluntary motion are retained in their integrity and there is little to indicate the presence of such an extensive lesion or the removal of so large a part of the brain. .- d yet. notwith- standing this ajiparent absence of physiological symptoms, I could perceive a very decided altera- tion in the animal's character and behavior, though it is dillicult to state in precise terms the nature of the change. The animals operated on were selected on account of their iiitelligint character. After the operation, though they might seem to one who had not compared their present with the past fairly up to the average of monkev intelligence, they had undergone a con- siderable psychological alteration. Instead of, as before, being actively interested in their sur- roundings, and curiously prying into all that came within the field of their observation, they remained apathetic or dull, or dozed off to sleep,