510 



SENSIBILITY. 



ment which is necessary to give precision 

 to our muscular efforts. This sense comes 

 greatly in aid of that of touch, and of those 

 powers which we derive from the sense of 

 touch. 



It admits of question whether this sense 

 really requires the presence of true nerves of 

 sensation in the muscles, and whether it may 

 not be due to the reaction of the muscular force 

 upon the proper muscular or motor nerves, 

 through which, by reflection at the centre, the 

 centre of sensation becomes affected. (See 

 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 shall have been withdrawn ; or the sensation 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 element 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 nerves of 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 afjjgcted 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 commissural 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, 

 HEARIN T G, SMELL, TASTE, TOUCH, VISION. 



(R. B. Todd.) 



SENSIBILITY. (Fr. Setia&ilite ; Germ. 

 Empjindlichkeif). This term, like Sensa- 

 tion, should be limited to signify the power 

 which any organ or tissue of the body has, of 

 causing changes inherent or excited in it to 

 be perceived and recognised by the mind. 

 The greater this power is in any tissue or 

 organ, the more sensitive it is, the greater 

 the sensibility of the organ or tissue ; the less 

 this power is, the less the sensibility of the 

 organ, &c. 



Sensibility, like Sensation, involves the 

 power of affecting the mind through the body; 

 but as the mind, of its own mere motion, may 

 excite the centre of sensation, so, by directing 

 the attention specially to some particular 

 tissue or organ, it may create a sensation 

 which, will be referred to that part, and which, 

 by frequent repetition, may assume the nature 

 of pain. No doubt many instances of hys- 

 terical pain are greatly aggravated by the 

 mind being constantly directed to, and dwell- 

 ing upon, the painful part. 



The term Sensibility is sometimes con- 

 founded with Irritability, especially by Psycho- 

 logical writers. Haller has, with great pre- 

 cision, laid down the distinction between these 

 two properties of tissues in the following 

 words : 



" Irritabilem partem corporis humani dico, 

 qua? ab externo aliquo contactu brevior fit ; 

 valde irritabilem, quae a levi contactu, parum 

 quae a valente clemum causa in brevitatem 

 cietur. Sentientem partem corporis humani 



