The fourth chapter describes the sensorium commune, its functions, and its seat. Ilere it is that Prochaska has put fomard his views respecting reflex actions. External impres sions, which are made upon sensitive nerves, are propagated with great velocity throughout their entire length to their origin, where, (to use his own plirase,) when they have arrived, they are reflected according to a certain law, and pass into certain and responding (certos ac respondentes) motor nerves, by which again being very quickly propagated to muscles they excite certain and determinate inovements. This place, lie says, in which, as in a centre, nerves of sense and of motion meet and commu nicate, and in which " the impressions of sensitive nerves are reflected into motor nen-es," is called, by a term already received by most physiologists, " the sensorium commune." Having referred to the various views of different physiologists its to the seat of the sensorium coininune, he expresses his own opinion, that the sensorium commune, properly so called, extends throughout the inedulla oblongata, the crura cerebri and cerebelli, a part of the optic thalami, and the entire spinal cord,—iii a word, as fitr as the origins of the nerves extend. That the sensoriuin cominune extends to the spinal cord is shown by those movements which con tinue in animals after decapitation, which can not be effected without the cooperation of nerves which arise froni the spinal cord ; for if a decapitated frog be pricked, not only does it retract the stimulated part, but also it creeps, and leaps, which could not be done without the consentancous action (absque consensu) of sen sitive and motor nerves, the seat of which cou sentaneous action must be in the medulla spi nal is, superstite sensorii communis parte.
That Prochaska vievved these acts as purely physical in their nature, is apparent from his statement, that they take place under peculiar laws, written, as it were, by nature on the me dullary pulp of the sensorium. The general law, however, whereby the sensorium com mune reflects sensorial into motor impressions (impressiones sensorias in motorias retlectit) is our preservation; so that certain motor impres sions should succeed to such external impres sions as inight be injurious to our bodies. In illustration he refers to certain acts of this class, such as, irritation of the mucous membrane of the nose creating a violent act of expiration (sneezing) to expel the offending material from the nostril; the spasmodic closure of the glottis when -a particle of food or a drop of fluid touches it, or the act of winking excited by the finger being brought close to the eye.
Prochaska points out that these reflex actions may take place with or without consciousness (vel anima vel vero animl conscii1). In proof of this occurrence without consciousness he refers to certain acts which are observed in apoplectic patients, to the convulsions of epi lepsy, and to certain actions in profound sleep ; all those actions which occur in decapitated animals he refers to this class, and regards them as being regulated by the remaining portion of the sensoriurn commune which is seated in the spinal cord. " Omnes istm actiones ex organism°
et physicis legibus, sensurio communi propnis, fluunt, suntque, propterea, spontanem ac au tomaticw." Actions, however, which the mind directs and moderates by its control, although the setworium commune may take its share in producing them, may be called animal, and not automatic.
The second paragraph of this chapter con tains an excellent discussion of the question, how far the anastomoses of nerves contribute to their mutual action upon each other, or whether that takes place only through the sensorium commune. On this subject Prochaska adopts the opinions of Whytt, who reg-arcled the nervous centre as essential to these actions, and in the next paragraph he enquires ..vhether nerves can establish any communication or consent with each other in their ganglia, and also discusses the use of ganglia, giving his assent, in some degree, to 'the doctrine which lie assigns to Unzer and Winterl,* that external impressions are capable of being reflected by ganglia as they are reflected in the sensorium commune, and that ganglia are particular centres of sensonal impressions (sensoria particularia). He sup poses that the action of the heart may be ex plained in this way through the impressions made by the blood upon its sensitive nerves which are reflected at the ganglia ft and he con cludes by admitting it to be probable that besides the sensorium commune which resides in the medulla oblongata, the medulla spinalis, 8te., there are sensona partindana in ganglia and anastomoses of nerves (concatenationibus ner vorum) which external impressions are re fiected, without their reaching the sensorium commune.
In the fifth and last chapter Prochaska dis cusses the animal functions of the nervous system. Ile shows that the soul, ens ineor porne prosaphr, uses the nervous system as an instrument, and that, in all animal functions, it is the prinelphan agens et detenninans. lie describes the principal parts into which the animal functions can be conveniently resolved, as perception, judgment, will, to which may be added imagination and memory. For the ex ereise of these he lays down that the joint and harmonious action of the mind and brain is necessary, and he assigns to each of them a different locality in the brain. In the last sec tion he again defines the animal actions, and distinguishes them from those which are de pendent on a physical exciting cause ; and argues against the Stalilian doctrine, which placed each movement and function of the body under the control of the soul.