Mfl 



SKN-A THIN- 



SENSITIVE PLANT 



corpus calloBum there lies a long convolution, the 

 gyrus fornicatus, the removal of which produces 

 complete low of tortile sensibility of the opposite 

 half of the body ; Inn in tin- 'motor area there 



are probably other centres, RK this is rarely d 

 without the production of peculiar modification)* of 

 sensation. The centre for hearing has been located 

 in the upper convolution of the temporal loin-, that 

 for smell and taste on the anterior end of the 

 temporal lolx>. An exhaustive consideration of the 

 dilliculties of the problem will be found in Foster's 

 Text-book of Phynology. 



Each of the special senses has its own peculiar 

 end-organ : the special endings of the olfactory 

 nerves in the upper part of the septum of the nose 

 for that of smell ; the retina in the eyeball for 

 sijjht ; the rods of Corti in the cochlea for hearing ; 

 the taste bulbs and the fibrils in the fungiform 

 papilla- in the tongue for taste; and the Pacinian 

 corpuscles and the special ramifications of the 

 cutaneous nerves in the epidermis for touch. The 

 integrity of these and of the special non-nervous 

 apparatus with which they are connected is neces- 

 -ary for the production of a sensation. Thus, the 

 transparent media of the eyeball, and the rods and 

 cones of the retina are all essential to the produc- 

 tion of a visual sensation. In proportion as they 

 are abnormal, the sensation is imperfect. Further, 

 each end-organ can be thrown into action only by 

 certain kinds of stimuli, and the nerves in con- 

 nection with them convey those impulses only 

 which give rise to their own special varieties of 

 sensation. The retina can only In- stimulated by 

 waves of light, never by those of sound, and the 

 optic nerve if stimulated directly can give rise to 

 visual sensations only. This law holds good also 

 for the cutaneous nerves, there being reason to 

 believe that the end -organs for touch are not the 

 same as those for temperature. It is probable 

 even that there are separate nerves for the per- 

 ception of heat and of cold. All parts of the end- 

 organs are not equally sensitive or callable of lieing 

 similarly affected by the same kinil of stimulus. 

 Thus, the same substance may appear sweet at the 

 anterior part and bitter at the posterior part of the 

 tongue. The retina is most sensitive at the yellow 

 spot, while the various colours are perceived over 

 areas which differ considerably. Tactile sensi- 

 bility varies exceedingly. The points of a compass 

 can lie felt as double at a distance of about ,<j 

 part of an inch at the tip of the tongue, of ^ of an 

 inch on the front of the tip of the forefinger, but 

 only at a distance of more than 2J inches on the 



The muscular sense is that by which we are made 

 aware of the position of any part of the body, 

 by which we gauge the amount of movement neces- 

 sary to affect any object or to overcome any resist- 

 ance. Many authorities deny the existence of 

 such a special sense altogether, and believe that it 

 is merely a form of ordinary tactile sense. But 

 that this in not so is frequently illustrated in a 

 disease of the spinal cord, locomotor ataxy, in 

 which there is, when the eyes are shut, a more or 

 lens complete absence of knowledge of the position 

 of the limbs and of the power of regulating their 

 movement*, although the cutaneous tactile sensi- 

 bility may be quite normal. It would appear likely 

 that the nerve endings connected with this sense 

 are situated in the muscles, tendons, and joints, 

 ami that these are stimulated by changes in move- 

 ment and mutual proxsure in these structures. 



The sensation of pain (or general sensibility) is 

 produced when pressure on a part, or when the tem- 

 perature of a body applied, exceeds certain limits. 

 What was at first a sensation of touch, or of heat 

 or cold, becomes replaced by a painful sensation. 

 It is very difficult to be certain whether there are 



special end-organs for the reception of such im- 

 pressions. It is quite beyond doubt that the 

 olfactory, optic, auditory, and gustatory nerves do 

 not transmit painful impressions, and it might be 

 supposed that the analogy would hold in the case 

 of the senses of touch and temperature. It is found 

 in some cases of disease that the prick of a pin 

 may be felt as a touch one or two seconds before 

 it becomes painful. But this is open to the ex- 

 planation that the delay in the latter case may 

 take place in the spinal cord owing to a different 

 path of conduction rather than to a difference in 

 the time that it takes to stimulate the end-organ. 

 Painful sensations may result from excessive stimu- 

 lation of a sensory nerve at any part of its course, 

 which would seem to point in favour of the non- 

 existence of special end-organs. AVith regard to 

 the paths by which these various impulses reach 

 the brain, we know ( if we except the fifth cranial 

 and the vagus nerves) that they reach the spinal 

 cord by the posterior roots of tin- spinal m 

 and that those impulses which produce tactile, 

 thermal, and painful sensations for the most part 

 (though this has recently been questioned) travel 

 up the side of the cord opposite to that at which 

 they entered, but their exact course is not certainly 

 determined. The path for the muscular sense im- 

 pulses is bv many regarded as lying in the ixisterior 

 columns of the same side (see SPINAL CORD). 



Within the medulla oblongata the obscurity as 

 to the upward sensory conducting tracts is even 

 greater than in the cord, not only in the case of 

 the senses above mentioned, but also of the sense 

 of hearing and taste. It is probable that the paths 

 are not continuous, but are frequently interrupted 

 by nerve cells, through the agency of which the 

 nervous impulses undergo successive elaborations 

 before reaching the cerebral cortex. The evidence 

 for the existence of special upward conducting 

 strands in the spinal cord, and the connections of 

 these, are fully considered under the article SPINAL 

 COLUMN AND SPINAL CORD. 



See Landois and Stirling's Physiology (for details of 

 investigations in various senses ) ; (juam's A natomy ( for 

 end-organs and nervous apparatus ) ; Foster's I'hiinioloyy 

 ( for general consideration of the problems involved ). See 

 also TABTK, NOSE, EYE, VISION, PEUCKPTIOM, Navou 

 SYSTEM, PSYCHOLOGY, VoL VIII. p. 474. 



Sensitive IMaiit. a name commonly given to 

 certain species of Mimosa (see MiMosK.t:), on 

 account of the peculiar phenomena of Irritability 

 (q.v.) which their leaves exhibit in their collapse 

 when touched or shaken. Numerous species of 

 Mimosa possess this property, and, indeed, most 

 of the species in a greater or less degree ; but those 

 in which it is most conspicuous are humble her- 

 baceous or half-shrubby plants. They have leaves 

 beautifully divided, again and again pinnate, with 

 a great number of small leaflets, of which the pairs 

 close upwards when touched. On repeated or 

 rougher touching the leaflets of the neighbouring 

 pinna; also close together, and all the junnic sink 

 down, and at last the leaf-stalk itself sinks down, 

 and the whole leaf hangs as if withered. If the 

 stem is shaken all the leaves exhibit the same 

 phenomena. After a short time the leaf-stalk 

 rises, and the leaflets expand again. On account 

 of this curious and interesting property some of 

 the sensitive plants are frequently cultivated in 

 our hothouses. They are generally treated as 

 annuals, although capable of longer life. .'/. 

 saiititim, one of the best- known species on the 

 continent of Europe, is a native of Brazil, with 

 prickly stems and leaf -stalks, and small heads of 

 rose-coloured flowers. M. pudica is the species 

 most commonly cultivated in British hothouses ; 

 it is a branching annual growing from 1 to 2 feet 

 high. M. auto, M. pudtbunda, M. palpitant, and 



