837 



ELEEP. 



SMELL. 



838 



the only ones which continue, and during that state even the organs 

 of animal life are rendered capable of renewed action by the organising 

 force which proceeds without the consciousness of the animal, though 

 accordant with a well-constituted plan and with reason." 



Little need be said of the uses of sleep. ' Nature's soft nurse,' it 

 invigorates body and mind when worn out by toil, and the occurrence 

 of sleep in the course of a disease is one of the most favourable signs 

 of returning health. After a night's sleep we are nearly an inch taller 

 than before ; the intervertebral cartilages, which had been compressed 

 by bearing the weight of the body during the day, having regained 

 their natural form and proportions. The powers exhausted by our 

 intercourse with the external world, recover themselves during sleep, 

 and our senses in the morning are alive to all impressions. It is how- 

 ever pre-eminently the rest of the brain, which, when fatigued by the 

 constant action of the mind, becomes incapable of continuing that 

 action, just as the eye, if long fixed upon one spot, ceases to perceive 

 any object distinctly. [NERVOUS SYSTEM.] 



Since then sleep is not a mere torpor of the system induced by 

 fatigue, long watching, or any external cause, but a natural state 

 whose periodical recurrence is essential to the harmonious performance 

 of our functions, it need excite in us no surprise to find that in certain 

 animals this condition lasts for a long time, even for months; and that 

 in it the activity of the organs of animal life is suspended more com- 

 pletely than in diurnal sleep. To this state the name of Winter Sleep, 

 or Hytxrnatiou, is applied. Hybernation occurs in some Mammalia, 

 in all the Amphibia, ;v.id in some of the Molluscous and Insect tribes, 

 liinls do not hybernate, and the vulgar notion with reference to the 

 sleep of the swallow in winter is erroneous. Hybernation is either 

 perfect or imperfect. In the former, of which the marmot affords an 

 instance, the lethargy is profound and undisturbed by any sense of 

 thirst or hunger, and the animals do not awake until the period of 

 sleep is completely past. In the latter, intervals of wakefulness occur, 

 during which the creatures rouse themselves and seek for food, as in 

 the case of many insects and spiders, also the hedgehog, bat, and 

 dormouse. The time during which hybernatiou continues varies 

 much : in some animals it lasts only four months, in others for five or 

 six ; but almost all awake either in March or April. The situations 

 which animals select for hibernation, and the position in which they 

 await it, differ, as might be expected, in accordance with their various 

 habits, but all seem to subserve one great purpose the preservation 

 of a moderate and equable temperature. Most creatures hybernate in 

 solitude, but some, as the marmot, pass the winter in company. 



The approach of winter-sleep is not sudden, but it comes on 

 gradually ; the activity of the animals diminishing as their sense of 

 hunger grows less keen. In some animals, as in the hedgehog, a 

 diminution or total loss of appetite precedes hybernation for some 

 weeks. When hybernation ig perfect, the senses become eo com- 

 pletely blunted, that severe wounds and electric shocks are insufficient 

 to rouse the animal. 



The phenomena of organic life go on far more sluggishly during 

 hybernation than in diurnal sleep. The pulsations of the heart sink 

 to a fourth or even a tenth of their natural frequency. The respira- 

 tion becomes slow, intermits frequently and for a long time, or even 

 becomes altogether imperceptible. An animal in this condition, if 

 placed in a pneumatometer, will produce no change in the air which 

 it contains, or may be kept for a considerable time in irrespirable 

 gases without sustaining any inconvenience. The temperature of the 

 body depends on the activity with which respiration and circulation 

 are carried on; hence, during hybernatiou it sinks greatly, and in 

 some animals is not higher than that of the surrounding atmosphere. 



Hybernation is not mere stupor from cold; all animals may be 

 benumbed by cold, but those which hybernate are comparatively few 

 in number. A moderately low temperature indeed is favourable to 

 hybernation, but instinct teaches the animals to defend themselves 

 from the cold, and their death results from exposure to its severity. 

 Suspended animation from cold is a morbid state; hybernation and 

 sleep are preceded by similar phenomena, and both terminate alike, 

 after a certain time, in renewed activity. 



If, lastly, we inquire what uses are answered by winter-sleep, we 

 shall see that it is a provision for the maintenance of life at a season 

 when those animals in which it occurs would be unable to obtain 

 tln'ir natural food. It serves likewise for their protection asrain-t a 

 degree of cold to which they could not bear to be exposed. Nor is it 

 merely against extreme cold that annual sleep and the instinctive 

 preparations of animals for that condition supply a defence, but 

 extreme heat is followed in some animals by a lethargy similar to 

 hybernation. Thus the Tanrec, or Rat of Madagascar, sleeps during 

 the height of summer. Changes in the system generally, and renewed 

 activity of the various functions, follow hybernation, as they do 

 diurnal eleep. It would lead us into discussions beyond our limits, 

 if we were to examine the various peculiarities of hyberuatiug animals, 

 or to inquire into what may be termed the proximate cause of annual 

 sleep. [HEAT, ANIMAL.] 



The revolutions of the seasons produce changes in Plants not unlike 



those which we have just noticed in hyberuuting animals. There is 



however a still closer analogy between diurnal sleep and the so-called 



'.f Pianta, a condition first discovered by Linmcus to be general, 



though some of the phenomena had long been noted in the tamarind- 



tree, and in some leguminous plants with pinnated leaves, natives of 

 Egypt. 



The attention of Linnseus was called to this phenomenon by the 

 following circumstance : Having sown some lotus seeds, he watched 

 the progress of the plants, and at length discovered upon one of them 

 two flowen. When evening came, he could not find the flowers again, 

 and supposed that some one had plucked them. On the following 

 morning he again observed them, and they once more disappeared at 

 evening. He then examined the plants with care, and saw that at 

 evening the leaflets had approached each other, and thus concealed 

 the flowers from view. Struck by this fact, he took a lantern in 

 his hand and visited the flower-beds, when to his surprise he found 

 the appearance of all things changed, and thus discovered the sleep 

 of plants. 



As night approaches, flowers close, the leaves of plants become 

 more erect and fold themselves together, while vitality seems to retire 

 from the periphery. Thus, during sleep the leaves of the sensitive- 

 plant lose their peculiar sensibility, which retires to the petiole. With 

 the approach of night too an important change takes place in the 

 functions of plants, for instead of exhaling oxygen and absorbing 

 carbon from the atmosphere, as in the daytime, their action at night 

 is directly the reverse. 



The sleep of plants usually occurs at night time, owing to the 

 withdrawal of the stimulus of light, to which they are subjected 

 during the day. The experiments of De Candolle indeed have proved 

 that by producing artificial day and night it is possible to change the 

 time for the sleep and waking of plants. There must however be 

 some cause of sleep more intimately connected with their organism 

 than the mere withdrawal of light; for not only are there plants 

 which, like certain animals, sleep through the day and are awake at 

 night, but it has been ascertained that the leaves of plants kept con- 

 stantly in the dark open and close at regular intervals, as during sleep. 

 [MOTIONS ov PLANTS.] 



SLEEP OF PLANTS. [SLEEP.] 



SLEEPER [ELEOTBIS.] 



SLO'ANEA, a genus of Plants belonging to the natural order 

 Tiliacece, named by Plumier in honour of Sir Hans Sloane. The 

 leaves are large and alternate, the flowers are large, and the fruit as 

 big as chestnuts. The trees are not known to be applied to much use, 

 with the exception of 5. dentata, of which the wood is sometimes 

 employed for making canoes of a single piece. The inner bark is 

 astringent, and prescribed in dysentery. The fruit ia eaten. 



SLOE. (PRUNUS.] 



SLOTH. [BRAD7PUS.] 



SLOW-WORM. [BLIND Wonii; ORVET; SAURIANS; SCINCID.E.] 

 SLUG. [LIMAX ; GASTEROPODA.] 



SLUGS, SEA. [NUDIBRANCHIATA.] 



SMARAGDITE. [AMPHIBOLITE.] 



SIIEATHMA'NNIA, a small but beautiful genus of Plants belonging 

 to the natural order Passifloracece. The genus possesses a 1-leaved nec- 

 tarium, which is urceolate and surrounding the base of the stamens ; 

 the stamens are numerous, distinct, and seated on a Bhort column 

 with incumbent anthers ; the stigmas are peltate and 5 in number ; 

 the capsule is inflated, and 4-5-valved. The seeds are dotted. All the 

 species are upright shrubs, with white showy axillary flowers, and are 

 natives of Sierra Leone. Three species are recorded, S. pubescens, 

 S. lievigata, and 5. media. 



SMELL. The essential part of the organ of smell consists of th.j 

 expansion of the olfactory nerves, the first or most anterior of the 

 nerves from the brain, whose minutest branches are distributed just 

 beneath the mucous membrane of part of the nose. [NOSE.] 



The olfactory nerves descend from the under surfaces of the 

 olfactory bulbs [BHAIN] through the foramina of the cribriform plate 

 of the ethmoid bone. They are very numerous, and are densely 

 distributed in bundles and tufts in the mucous membrane covering 

 the upper part of the septum, the under surface of the cribriform 

 plate, and the inner surfaces of the superior and middle turbinated 

 bones, and of the cells immediately adjacent to them. 



All that is necessary for the perception of an odour is that the 

 scented particles (without undergoing any such changes as light does 

 in arriving at the retina, or sound on its way to the auditory nerve) 

 should come in contact with the surface under which the olfactory 

 nerves lie, with the force of rather more than an ordinary inspiration. 

 If the medium containing the odour be at rest, or be only gently 

 forced against the membrane, no impression is produced. 



In different animals the sense of smell is adapted chiefly to that 

 class of substances on which they feed. The Cwnii'ora, for example, 

 have an acute sense of the odour of animal substances, but, so far as 

 we can discern, none for that of vegetables ; and, on the other hand, 

 Hcrbivora are as clear in their perception, of the latter, and as nearly 

 insensible to the former. Man, as his food is mixed, so also is his 

 sense of smell adapted to both classes of substances, though for each 

 less acute than that of the animals that feed exclusively on the one 

 or the other. In the choice of food, which is the main object of the 

 sense of smell, man generally, though almost unconsciously, and 

 animals always, exercise the precaution of smelling, and they 

 instinctively form a judgment according to the impression received. 

 In eating also, much of that which is commonly attributed to the 



