TOUCANS. 



TOXODON. 



TOUCANS. [lUurnAsriD.E.] 



Tol'CH. The sense of touch belong* to the outward integument 

 of the body the akin, and it shared, in a minor and modified degree, 

 by porU of the mucous membranes, which, at the various orifices of 

 the body, are continuous prolongations of the same structure. By it 

 we hare the faculty of determining our immediate relations to material 

 objects, in regard of contact, temperature, and electrical excitement 



The nervom influence, or endowment, on which this sense depends, 

 appears in no enential particular to be distinguished from that which 

 gives common sensibility to the various deeper organs of the body. 

 The nerves which convey the specific impressions of touch, are asso- 

 ciated at their origin, or in their course, with others not reaching to 

 the surface of the body, under the common name of sensitive ; so that 

 if we trace such a one from the centre outwards, we find it (previously 

 to its termination in the skin, where it ministers to the special sense 

 of touch) supplying parts which have merely that common feeling, or 

 sensibility to pain, necessary to their own preservation. 



In order to that high development of the sense of touch, which we 

 notice in the human hand, or in the proboscis of the elephant, certain 

 anatomical peculiarities are required : 



1. An exposure of the largest possible number of points, each 

 endowed with sensibility, and capable of being recognised in the senso- 

 rium, as distinct and individual Such a structure is eminently illus- 

 trated in the papillary surface of the skin ; which presents a vast 

 number of minute evolutions, or papilla), every one furnished with its 

 own fibrillary nervous loop, and a minute inosculation of blood-vessels. 

 It is certain that each terminal loop is represented by a definite point 

 in the brain, that no two nervous fibrillee become blended or confused, 

 and hence that each papilla may originate a single and particular 

 sensation. Perhaps the strongest familiar evidence that can be adduced 

 for this fact, ia furnished in the tingling sensation (called ' pins aud 

 needles') which we produce in numbing the nerve of a part by pres- 

 sure ; the impression here made on the trunk of a nerve being referred, 

 according to a well-known physiological law, to its cutaneous extre- 

 mities ; so that when, for example, we strike the funny-bone, or other- 

 wise press the uluar nerve which lies there, the peculiar tingling is 

 felt in the akin, at the tips of the last two fingers : if this sensation be 

 analysed, we find it to consist in a pricking of innumerable minute 

 points, as though there were falling on the part a quick dense shower 

 of the finest needles : the minute points, HO made sensible to us, are 

 single papillae of the skin. This instance of subjective sensation is 

 probably the only mode in which the sensitive faculty of the skin can 

 be made known to us ; for obvious reasons prove it impossible to 

 apply to the organs of touch the same precise admeasurements as we 

 may use in ascertaining the power of the eye or of the ear. It appears 

 however that the tactile power of the skin varies for different parts in 

 a very much greater degree than can be accounted for by differences 

 in nervous supply. Professor Weber has measured the power of dis- 

 tinguishing distances in different parts of the skin, with the view of 

 affording means to compare their relative degrees of endowment, lie 

 conducted bis experiments by touching the surface with the points of 

 a pair of compasses (blunted with pieces of cork), and observing how 

 close their arms might be brought together, and still be felt each 

 distinctly. He found that the point of the tongue could distinguish 

 them, as two impressions, when distant only half a line from each 

 other; and that in the middle of the back a separation of thirty lines 

 was necessary to the distinction. Thus, while the tactile endowment 

 of a part is in proportion to the minuteness and density of its papil- 

 latiou, it is evident that the applicability aud development of this 

 property will depend on habit and attention and practice, by which we 

 acquire the power of appreciating minute differences of impression. 



2. In addition to the anatomical arrangement just described, the 

 sense of touch requires for iU perception that a muscular apparatus 

 should be connected with the sentient surface, by means of which 

 this may adapt itself to the superficies of bodies, in order to explore 

 their outline, span their dimensions, or probe their texture. And we 

 find accordingly in those organs which are most tactile, the hand, 

 the tongue, the lips of the human subject, the snout, proboscis, or 

 tentacles of lower animals, that complicated muscular motions 

 belong to the part, and render it a more available instrument of 

 exploration. The importance of this addition becomes manifest, if 

 we apply a foreign object to any plane surface of the body (to the 

 front of the fore arm, for instance), and hold it there without pressure 

 or motion. The only sensation so conveyed is one of indefinite con- 

 tact : without pressure we know not its consistence ; without successive 

 and exploring movement we cannot ascertain iU outline or level 



(Kirke., Handbook of I'hyiMogy ; Valentin. Tcrtbeok of Phytiology. 



TOUCH-ME-NOT. [Ijn'iTWns.1 



TOUCHSTONE. [FMXTT SI-ATE.] 



TOURACOS. [MuwriuuiDA] 



TOURMALINE (Sdul), a Mineral occurring crystallised. Primary 

 form a rhomboid; secondary forms, prisms with from 3 to 12 sides, 

 terminated by very irregular summit*. Cleavage parallel to the faces 

 of the primary form. Fracture uneven, couchoidal. Hardness : 

 scratches glass easily. Electricity : according to Hauy the transparent 

 varieties when heated become electric ; the termination of the prism 

 which presents the greater number of laces becoming positive, while 

 the other end becomes negative. Colour white, brown, blue, yellow, 



green, red, and black. The blue variety is called Indieolilf, and the 

 red Kulxllite. Schorl is a name formerly given to black tourmaline. 

 Lustre vitreous. Transparent, translucent, opaque. Specific gravity 

 3-060 to 3-070. Before the blow-pipe all the varieties fuse readily 

 with borax, producing at first slight effervescence. 



Tourmaline occurs most commonly in primary rocks, especially in 

 granite, gneiss, and mica slate. The red variety, or rubellite, is found 

 in Siberia ; the blue, or indicolite, at Uto ; the green in Brazil ; the 

 black in Devonshire and many other places. The composition of 

 tourmaline is extremely various. That of the red appears, from the 

 analysis of Vauquelin, to bo the simplest, consisting of 



Silica 



Alumina 



Soda 



Oxide of Manganese 



42 

 40 

 10 

 7 

 99 



Some varieties contain also potash, lithia, lime, magnesia, oxide of 

 iron, and boracic acid. The proportions of these constituents are 

 extremely variable, and no one variety contains all the enumerated 

 substances. The black tourmaline from Devonshire yields nearly 

 ] 9 per cent of oxide of iron, and that of Eibenstock nearly 24 per 

 cent of the same oxide. 



The word Tourmaline is a corruption of the name in Ceylon, 

 whence it was first brought to Europe. Lyncurium is supposed to 

 be the ancient name for common tourmaline; and the red variety 

 was probably called Hyacinth. 



The red tourmalines, when transparent and free from cracks, are 

 of great value, and afford gems of remarkable beauty. They have all 

 the richness of colour and lustre belonging to the ruby, though 

 measuring an inch across. A Siberian specimen of this variety, now 

 in the British Museum, is valued at 500?. The yellow tourmaline, 

 from Ceylon, is but little inferior to the real Topaz, and is often sold 

 for that gem. The green specimens, when clear and fine, are also 

 valuable for gems. 



Tourmaline, on account of its property of polarising light, is used 

 largely in the manufacture of polarising instruments. 



(Dana, Manual of Mineraloyy.) 



TOURNEFO'RTIA, a genus of Plants belonging so the natural 

 order Boraginacece, so named by Linnaeus in honour of Joseph Pitton 

 de Tournefort It is characterised by having a salver-shaped or rotate 

 corolla, of which the throat is naked ; the stamens included within 

 the tube of the corolla ; the stigma is peltate, and the fruit consists 

 of a berry which contains 4 1 -seeded nuts. The species are about 50 

 in number, forming small shrubs or herbs, diffused through the hot 

 parts of the world, as the West Indies, South America, Indian islands, 

 and India. 



TOXICODENDEON. [Ruus.] 



TO'XODON (rrftor, a bow; oSoi/j, a tooth from the curvature of 

 the teeth), Professor Owen's name for an extinct genus founded on 

 To.rodon Platentit. 



Mr. Darwin, during his sojourn in Banda Oriental, having heard of 

 some giants' bones at a farm-house on the SarandU, a small stream 

 entering the Rio Negro, rode there and purchased, for the value of 

 eigbteen-penoe, the cranium now in the Museum of the Royal College 

 of Surgeons in London, and the subject of Professor Owen's description. 

 The people at the farm-house told Mr. Darwin, that the remains were 

 exposed by a flood having washed down part of a bank of earth. When 

 found the head was quite perfect ; but the boys knocked the teeth out 

 with stones, and then set up the head as a mark to throw at Mr. Dar- 

 win found a perfect tooth, which exactly fits one of the sockets in this 

 skull, embedded by itself on the banks of the Rio Tercero, at the 

 distance of about 180 miles from the farm-house. Near the Toxodon 

 Mr. Darwin found what he terms the fragments of the head of an 

 animal rather larger than the Horse, and which he describes as having 

 some points of resemblance with the Toxodon and others, perhaps 

 with the Edentata. These fragment*, now also in the museum of the 

 College, Professor Owen has ascertained to belong to the lower jaw of 

 the %r<x/on. Mr. Darwin observes that the remains appeared to 

 fresh as to render it difficult to believe that they had lain buried for 

 ages underground. The bone contained so much animal matter, that 

 when heated in the flame of a spirit-lamp it not only exhaled a very 

 strong animal odour, but likewise burned with a slight flame. 

 ('Journal') The place where the remains were found is about 120 

 miles N.W. from Monte Video, and the cranium was embedded in a 

 whitish argillaceous earth forming the banks of the Sirandis. 



The first notice of this discovery appears in the ' Proceedings of the 

 Geological Society of London,' in a paper by Professor Owen, from 

 which we take the following brief account : 



The cranium measures 2 feet 4 inches in length, and 1 foot 4 inches 

 in extreme breadth. The form of the skull is elongated, depressed, 

 and chiefly remarkable for the strength and wide expanse of the 

 zrgomatic arches, and the aspect of the occipital foramen and occipital 

 region of the skull, which slopes from below upwards and forwards. 

 The maxillary portion of the skull is compressed laterally, narrow, 

 and with large intermaxillary bones slightly dilated at th'-ir 

 extremity. 



The teeth consist of molars and incisors. The latter are four in 

 number in the upper jaw, the two middle ones very small, the two 



