termed the olfactory epithelium. It is composed of non-ciliated columnar cells, among which are scattered narrow, rod-like 'olfactory cells' with large oval nuclei. Cutaneous sensibility includes common sensation, and is sometimes indiscriminately classed with a special sense-that of touch; it is by means of cutaneous impressions that we are made conscious of the consistence, texture, and temperature of external objects, and it is partly by cutaneous sensation, partly by muscular sensation,' that we ascertain and compare their weight. As tactile organs, the skin of the palm of the hand and that of the sole of the foot play a most important part in our guidance: the varying pressures of the body on the plantar surface guide equilibration and locomotion; we touch or feel objects by means of the hand and finger-tips, and we consciously utilise the sensations so acquired in our knowledge of objects and in our skilled move ents. The two most noteworthy subjects in connection with cutaneous sensibility are:-(1) its differences at different parts as ascertained by Weber's compass method; (2) the question of specific heat nerves' and 'cold nerves,' as investigated by Blix and by Goldscheider. The nerve-ends to which tactile function is attributed are the touch-corpuscles of Wagner; these, in the specially tactile area, e.g. the finger-tips, are set in ranks occupying rows of papillæ beneath the edges of the skin, forming thus what may be collectively regarded as a tactile organ. Each touch-corpuscle is a small oval glomerulus, 50 to 200 μ long, formed of the branched and tangled ends of a medullated nerve-fibre, held together and surrounded by connective tissue. They are unequally distributed over the surface of the body, being numerous and crowded at the tips of the fingers, few and scattered on the arms, legs, and trunk, and our discriminative tactile power by means of palpation is in evident relation with this inequality of distribution. We can 'feel' things better with the finger-tips than with any other part of the body, although these have by no means the most sensitive' skin. Tactile sensation, although less obviously a special sense than vision or hearing, is different from common sensation, but unless we are on our guard there is risk of confusion, owing to the ambiguity of such familiar terms as 'feel and 'sensitive.' We feel the pulse with the finger-tips, but to feel whether it rains or not we hold out the hand with the palm downwards; if the palm of one hand and the back of the other are exposed together to a gentle drizzle, the droplets may be distinctly felt on the latter, while they are quite imperceptible on the former. We have already encountered a somewhat analogous distinction in the case of the retina; the yellow spot, the region of most distinct vision, is not the most sensitive part of the retina (p. 453). A measure of tactile discriminative power is obtained by finding the greatest distance at which two points of a pair of compasses applied to the skin are felt single, or the smallest distance at which they are felt double. Tested by this method (Weber's), the following are some average values in milli By minute investigation of the skin, supplemented by sub jective introspection, some remarkable conclusions have been arrived at, viz. that the skin contains many kinds of nerve-fibres and endorgans, separately subserving impressions of pressure, of pain, and of temperature. Of these investigations, the most noteworthy is that of Goldscheider, to the effect that different spots of skin are respectively excitable by heat and by cold, and that the nerve-fibres leading off from such cold' spots and 'heat' spots are specifically different from each other, as well as from the nerve-fibres leading off from 'pressure-spots,' and from 'painspots,' in accordance with the law of specific nervous energies. 472 PAGE CHAPTER XIV THE SPINAL CORD AND BULB 472 Physiological anatomy: White and grey matter-Nerve-fibres and nervecells-White columns and vesicular columns-Segmental centres. 77 The spinal cord as a conductor: Nerve-roots-Their functions - Effects of section and of excitation-Recurrent sensory fibres. 480 Paths of motor and of sensory impulses: Three kinds of data-experimental, pathological, and developmental-Complete transverse division-Hemisection-Longitudinal division-Two hemisections-Direct excitability of spinal cord-Stenson's experiment-Functional excitability-Rate of conduction-Tracts of degeneration-Development. 489 The spinal cord as a centre: Reflex action-Automatic action-Psychical action Summation of stimuli-Diffusion of stimuli-Inhibition of stimuli. Time of reflex action-Effects of strychnia-Tetanus-Clonus-'All or nothing '-Diffusion-Lost time. 494 Centres of the spinal cord and bulb: The general notion of special centresTheir enumeration. 496 Co-ordinated movements-Muscular tonus Tendon-reflex -Equilibration -The semicircular canals. THE CRANIAL OR BULBAR NERVES 502 Their bulbar nuclei of origin-Summary of their functions. The spinal cord and bulb. Structure.-The spinal cord consists anatomically of white matter and of grey matter; histologically, the white matter is chiefly composed of nerve-fibres; the grey matter is chiefly composed of nerve-cells and of nervefibrils; physiologically, the spinal cord is a nerve-centre by virtue of the cells of the grey matter, a conductor of nervous impulses by virtue of the nerve-fibres of the white matter, and in less degree by virtue of the nerve-fibrils of the grey matter. Nerve-fibres in the white matter are medullated, vary greatly in diameter, and have no sheath of Schwann. Small fibres (5 μ) predominate in the posterior columns, large fibres (15 μ) in the cerebellar tract. Nerve-fibrils in the grey matter are for the most part the axis-cylinders of nerve-fibres and the branching processes of nerve-cells. Nerve-cells in the grey matter vary greatly in size; 'large' cells (60 to 130 μ in diameter) are most 1. Cell of ant. cornu. 2. Cell of Clarke's column. 3. Solitary' cell of post. cornu. 4. Large nerve-fibre. 5. Ganglion-cell of posterior root. Drawn to the same scale, viz. x 200 diameters. numerous in its anterior parts, and are connected with nervefibres of the anterior roots of the spinal nerves; 'small' scattered cells (20 μ in diameter) in the posterior parts of the grey matter are probably in relation with nerve-fibres of the posterior roots of the spinal nerves. The supporting tissue in which the true nervous elements are embedded is called the neuroglia; it is of a fine fibro-cellular structure resembling adenoid tissue, but, chiefly on account of its resistance to artificial digestion, it is classed with keratin, and supposed to have originated from the epiblast. It persists in greatest abundance in the substantia gelatinosa, which surrounds the central canal of the cord, and caps the tip of the posterior cornu. The central canal itself preserves through life a permanent vestige of its original formation by an infolding of the epiblast, in the form of a lining of ciliated epithelium. Nerve-cells and nerve-fibres are accumulated into longitudinal groups and strands, forming the various columns of the cord; strands of fibres form the white columns, longitudinal groups of cells form the vesicular columns. Grooves or fissures divide the white columns more or less superficially from each other into anterior, posterior, and lateral columns. The relative situation of these several parts will be best understood by The term 'lateral' is here used in its original anatomical sense, and is comprehensive of the pyramidal and cerebellar tracts, as well as of the 'lateral columns' of neurologists. studying them in a transverse section (fig. 258), and an idea of their longitudinal extension may be gathered from the diagram given in fig. 259. The white columns and the anterior vesicular groups extend through the whole length of the cord, and the latter are most prominent in the cervical and lumbar enlargements, the spinal centres from which the great nerves of the anterior and posterior extremities respectively take origin. The intermedio-lateral or lateral tract and the postero-median group of cells, known as Clarke's posterior column, are elongated islands rather than continuous columns of cells. They have their greatest development in the thoracic region, in which the anterior FIG. 258. DIAGRAMMATIC TRANSVERSE SECTION OF THE SPINAL CORD, × 6, on a level with the eighth thoracic nerve. (After Schwalbe.) columns of cells are most scanty; in this region Clarke's column forms what is known as the dorsal nucleus. Similarly-situated islands of cells are found in the cervical and in the sacral regions, known as the cervical nucleus and as Stilling's sacral nucleus. In the upper cervical region the lateral nucleus forms the nucleus of origin of the spinal accessory nerve; in the bulb it is represented by the antero-lateral nucleus (=the chief vasomotor centre ?). At the bulb, in consequence of the opening out of the central canal into the fourth ventricle, and of the pyramidal decussation, the columns of the cord diverge and interlace, and its central grey |