3. The vigour of the nervous actions bears a direct relation to that of the nervous force and to the power of the exciting stimulus. The actions of the nervous system will be greater and more vigorous in proportion as the vis ner vosa may be more active (mobilior) and the stimulus more efficacious ; 013 the other hand, the nervous force will be more sluggish and the stimulus less effective, where the nervous actions are more languid. A less stimulus is sufficient for a more active vis nervosa, as the application of a stronger stimulus may com pensate for a more sluggish vis nervosa, yet an equal effect may be produced in the nervous actions. The nervous force, however, is not equally susceptible to every kind of stimulus; sometimes it obeys one more than another, although both may appear equally powerful : nay, sometimes it experiences a more powerful effect from the stimulus which may seem the mildest. According to Haller, the heart and intestines are more powerfully stimulated to contract by air blown into them than by water or by any poison ; on the contrary, a drop of water let fall into the trachea excites violent cough, whereas air passes through it in breath ing as if unfelt by it.
4. The nervous force is augmented by va rious circumstances. Among these he enume rates age—at an early age the vis nervosa being greater than at a more advanced period of life— clinwte and disease.
5. On the other hand, the vis nervosa may be depressed or diminished by all causes which depress the powers of life, by the direct appli cation of opium and other sedatives to the nervous matter.
6. " Vis nervosa est divisibilis et absque cere bro in nervis subsistit." In illustration of this law he adduces the instances of nerves remain ing excitable after they have been separated from the cord or from the brain ; also the exci tability of paralytic limbs by the electrical stimulus. And, he states, the vis nervosa not only remains for a long time in the spinal cord and nerves which have been sepamted from the brain, but even in nerves which never bad any connection with the brain, as is shown by the acephalous fcetus, which, without a brain and by the sole force of the nerves and medulla spinalis, if this be not deficient, lives the full time in the uterus of the mother, is nourished, grows, and, when it comes into the light, shovvs often no obscure signs of life. To this law he attributes the persistence of the rhythmical ac tion of the heart after the decapitation of ani mals.
7. Idiosyncrasy is a peculiar affection of the nervous force. Among the examples of idio syncrasy he enumerates, fainting at the sight of blood, the uneasiness and even terror produced in some persons by the exhalations from a cat, which may be in the same room, although un seen; fainting from the perception of particular odours.
In his third chapter Prochaska proceeds to examine the functions of nerves. He describes
the mode of action of nerves, their power of receiving impressions with great facility, and of propagating them with the greatest velocity either to the centre or to the periphery. This power he calls the vis nervosa of nerves, which also may be called the sensibility or mobility of nerves, and to which Unzer had given the name corporealsense without concomitant per ception. And he shows that this power is in ,. _ I.,* hcrent in the medullary pulp of the nerves, and is not simply derived from the brain, but that a certain cohesion of the medullary pulp of the nerves is necessary for the developement of the vis nervosa, because if by compressing a nerve strongly we injure its medulla, so as to disturb the connection of its particles, the nervous force ceases in that part of the com pressed nerve, nor are impressions propagated further by it, nor if that part of the nerve be stimulated can sensation or motion be pro duced.
Although, he says, a nerve is necessary for sense and motion, it is not it alone which feels or moves ; it feels by the brain, which, when an impression made upon a nerve is conveyed to it, represents that impression to the mind; and a nerve causes motion hy the muscle when an impression, communicated to the nerve, de scends to the inuscle and excites it to motion. Ile concludes thus: " l'ar itaque nervi, in sensu et motu ciendo, est officinal, nimirum im pressionem stimuli recipere, et per totam snam longitudinem celerrim6 propagare, qme duni ad cerebrum pervenit, sensus perceptionem cansat, dum vero ad musculum, ejus contrac tionem ciet." Prochaska recognises the influence of the nerves upon the bloodvessels, and ascribes va rious familiar phenomena to this influence, either excited by direct contact of the nerves of the part, or, if the nerves be indirectly affected through the brain (si isti nervi non immediate, sed mediante cerebro afficiantur). Thus he refers to redness of the skin of the face occasioned by exposure to a cold wind, redness of the conjunctiva caused by some irri tant, erection of the nipple of the breast by titillation, erection of the penis by similar means or through inental emotion, blushing, &c. Ile puts forward the notion that the aug mentation of the nervous force in any part causes an attraction of fluids to that part, as sealing-wax, when rubbed with cloth, becomes electrical and attracts various small particles. To a similar attraction of fluids he ascribes muscular action and many other phenomena, such as the menstrual flux, the action of the iris, &c. lie also discusses the question whe ther the nerves have any power over the secre tions, whether they contribute in any way to the production of animal heat, and how far they are necessary to nutrition.