NERVOUS SYSTEM, PHYSIOLOGY OF.) All nerves of sensation are excitors of mo tion under certain circumstances, but especi ally when they are organised at their periphe ral distribution in a peculiar manner.
Objective and subjective sensations.— In the ordinary mode of exciting sensations the pre sence of an object is necessary. This object creates an impression on the peripheral parts of the sensitive nerves ; and the change caused by this impression, being duly propagated to the centre of sensation, is perceived by the mind. Thus is produced what some meta physicians call an objective sensation.
Such sensations are durable or transient, according to the force of the primary impres sion. The mind may continue conscious of the sensation long after the exciting object shall have been withdrawn ; or the sensation having ceased, the mind may recall it, with more or less exactness, without the renewal of the original stimulus. This is one form of subjective sensation, in which a mental act can develope a sensation independently of any present object, but resembling a previously experienced objective sensation. Other forms of subjective sensations are caused by phy sical changes in nerves themselves, or in those parts of the centres in which they are implanted. These changes are caused by alterations in the quantity, but more fre quently in the quality, of the blood, the deficiency in some of its staminal principles, or the presence of some abnormal elenient in it, or by modifications in the nutrient actions of the nerves or nervous centres. Subjective sensations of this kind are those most com monly met with. As examples of them we may refer to the motes or flashes of light occasioned by disturbed conditions of the retina, mechanically or otherwise ; or of the optic nerve ; or of those parts of the en cephalon in which the optic nerve is im planted ; tinnitus aurium, or singing in the ears, resulting from some analogous affections of the auditory nerve, or of the parts of the brain with which it is connected ; pains, or feelings of tingling or creeping in the limbs (formication).
Reflex sensations. — The physical change developed in the production of an objective sensation at one part may give rise to what may be compared to a subjective sensa tion in another and a remote part of the body. The irritation of a calculus in the bladder will give rise to pain at the end of the penis, or to pains in the thighs. The ob ject by which the irritation of the bladder is excited cannot exercise any direct influence on the nervesof the penis or of the thigh; through the nerves of the bladder it excites that por tion of the cord in which both the vesical nerves and the nerves of the penis and of the thigh are implanted, and thus the latter nerves are stimulated at their central ex tremities through the influence of the peri pheral stimulation ; in other words, the phy sical changes excited in the first are reflected into the second.
Sometimes distant and apparently wholly unconnected parts may be affected in this way. Thus irritation of the ovary will cause pain under the right or left mamma ; stimula tion of the nipple, whether in male or female, gives rise to peculiar sensations referred to the genital organs ; ice suddenly introduced into the stomach will cause intense pain in either supra-orbital nerve ; acid in the sto mach is apt to cause a similar pain, which may be very quickly relieved by the neutralisation of the acid. Phenomena of this kind imply some closeness of connection between the nerves of the sympathising parts in the centre, probably by means of conunissural fibres con necting the respective points of implantation of the nerves with each other.
For further remarks on the subject of this article see NERvous SYSTEM, PHYSIOLOGY OF ; and the articles on the Senses,