120 EXTEBNAL SENSATIONS. 



and vision. All these have some properties in common. They are 

 situate at the surface of the body, so as to be capable of being acted 

 upon with due facility by external bodies. They all consist of two parts : 

 the one, physical, -which modifies the action of the body, that causes 

 the impression; the other nervous or vital, which receives the impres- 

 sion, and conveys it to the brain. In the eye and the ear, we have 

 better exemplifications of this distinction than in the other senses. The 

 physical portion of the eye is a true optical instrument, which modifies 

 the light, before it impinges upon the retina. A similar modification 

 is produced by the physical portion of the ear on the sonorous vibra- 

 tions, before they reach the auditory nerve ; whilst in the other senses, 

 the physical portion forms a part of the common integument in which 

 the nervous portion is seated, and cannot be easily distinguished. Some 

 of them, again, as the skin, tongue, and nose, are symmetrical, that is, 

 composed of two separate and similar halves, united at a median line. 

 Others, as the eye and ear, are in pairs; and this, partly perhaps, to 

 enable the distances of external objects to be appreciated. We shall 

 find, at least, that there are certain cases, in which both the organs are 

 necessary for accurate appreciation. 



Two of the senses vision and audition have, respectively, a nerve 

 of special sensibility; and, until of late years, the smell has been 

 believed to be similarly situate. In the present state of our knowledge, 

 we cannot decide upon the precise nerve of taste, although it will be 

 seen that a plausible opinion may be indulged on the subject. The 

 general sense of touch or feeling is certainly seated in the nerves of 

 general sensibility connected with the posterior roots of the spinal 

 nerves and the fifth encephalic pair ; and according to some, 1 in the 

 glosso-pharyngeal and pneumogastric nerves. The other senses seem 

 intimately connected with one nerve of general sensibility, the fifth 

 pair. This is especially the case with those senses that possess nerves 

 of special sensibility ; for, if the fifth pair be cut, the function is 

 abolished or impaired, although the nerve of special sensibility may 

 remain entire. 



Being instruments by which the mind becomes acquainted with ex- 

 ternal bodies, it is manifestly of importance, that the senses should be 

 influenced by volition. Most of them are so. The touch has the plia- 

 ble upper extremity, admirably adapted for the purpose. The tongue 

 is movable in almost every direction. The eye can be turned by its 

 own immediate muscles towards objects in almost all positions. The 

 ear and the nose possess the least individual motion; but the last four, 

 being seated in the head, are capable of being assisted by the muscles 

 adapted for its movements. 



All the senses may be exercised passively and actively. By direct- 

 ing the attention, we can render the impression more vivid; and hence 

 the diiference between simply seeing or passive vision, and looking 

 attentively ; between hearing and listening ; smelling and snuffing ; 

 touching and feeling closely. It is to the active exercise of the senses, 

 that we are indebted for many of the pleasures and comforts of social 

 existence. Yet, to preserve the senses in the vigour and delicacy, 



1 Longet, Traite de Physiologic, ii. 176, note. Paris, 1850. 



