SPINES. 



[ 710 ] SPIRAL STRUCTURES. 



vertebra. The cord is composed of white 

 substance, grey substance, and blood-vessels 

 (PI. 52. fig. 7). 



The white substance («) is mostly ex- 

 ternal, and forms the antero-lateral and 

 posterior longitudinal columns, besides the 

 transverse anterior commissure at the end of 

 the anterior longitudinal fissui-e. It is com- 

 posed of nerve-fibres of large and medium 

 size, of connective tissue, and blood-vessels. 

 The grey substance (c) is internal, and in 

 transverse section of the cord presents a 

 shape which may be roughly compared 

 with that of the letter H. On either side 

 there is an anterior and posterior horn or 

 cornu; and the median line of the letter is 

 represented by a soft grey commissm'e per- 

 forated by a canal, which is central and 

 lined with epithelium, SuiToimded on all 

 sides by the white substance, the anterior 

 and posterior spinal nerves reach their re- 

 spective cornua by crossing at different angles 

 along two lines on either side of the ge- 

 neral direction of the nerves of the white 

 substance. 



The grey substance contains a large num- 

 ber of very fine nerve-fibres, which are 

 united in a plexiform manner with nerve- 

 or ganglion-cells ; most of these fibres pass 

 outwards and become spinal nerves, belong- 

 ing then to the white substance or to di- 

 stinct nerves. There is moreover granular 

 matter with highly refractive globules, and 

 extremely small nucleated cells surrounding 

 the nerve-fibres and ganglion-cells. Much 

 connective tissue exists in the neighbourhood 

 of the central canal and in the posterior 

 cornu. 



The examination of the spinal cord and 

 the brain is so difficult and not likely to be 

 the subject of general investigation, that 

 we must refer to the Bibl. for further de- 

 tails. 



BiBL, Todd's Ci/cl. An. ^- Phys. ; Lockhart 

 Clarke, Phil. Tr. 1858, 1868 ; Strieker's 

 Hist. ii. ; Frey, Hist., and the Bihl. 



SPINES OF Animals. — These are pro- 

 perly stout rigid and pointed processes of 

 the integument, formed externally by the 

 epidermis, and internally of a portion of the 

 cutis or corresponding structure ; but the 

 temi is frequently ajiplii'd to stout rigid and 

 puintf d processes of the epidermis only. 



See Hairs, and the notices of the struc- 

 ture of the integument under the heads of 

 the various classes. 



SPIRACLES or Stiumata of animals. 

 — ^The external orifices of the tracheaj of 



Insects and Arachnida. The respiratoiy 

 tubes of these animals have no communica- 

 tion with the mouth, but terminate exter- 

 nally in orifices situated upon the surface of 

 the thorax or abdomen. These are mostly 

 rounded or elliptical (PI. 35. figs. 3, 7, 8, 

 and 9 a), sometimes in the form of small 

 clefts, and are often furnished with a kind 

 of moveable valve, or bounded by a thick- 

 ened rim ; sometimes a sieve-like sti'ucture 

 (PI. 34. fig. 34) prevents the admission of 

 foreign bodies, or they are surrounded by 

 hairs or scales effecting the same purpose. 



They are often situated at the lateral 

 and upper portions of the abdomen, at the 

 posterior, lateral, and upper part of the 

 thorax, &c. 



See Arachnida, Insects, and the 

 genera. 



SPIRAL STRUCTURES of Plants.— 

 Among the most elegant of the microscopic 

 objects fm-nished by the Vegetable King- 

 dom are the various forms of the secondary 

 deposits upon the walls of cells, vessels, and 

 ducts, Sec, which present the appearance of 

 fibres coiled into perfect spirals, or of spiral 

 fibres either with the coils detached and 

 forming rings, or with the coils more or less 

 connected by cross pieces, producing a re- 

 ticulated structure. 



Under the head of Secondary Deposits 

 it is stated that tliis spiral-fibrous deposit 

 may be taken as the character of a group of 

 structm-es to be contrasted with those struc- 

 tures described as Pitted ; and that the 

 essential distinction in the nature of these 

 two groups lies in the greater extent to 

 which the primary wall is covered in the 

 pitted structures. This is not quite absolute 

 in reference to all spiral-fibrous structures, 

 as in the true unrollable spiral vessels and 

 similar organs the coils of the spiral fibres 

 are often closely in contact, although not 

 adherent to each other. It has been stated 

 that the various fonns of the open spiral, 

 annular, and reticulated deposits are modi- 

 fications of the simple close spiral ; but this 

 must be understood only in a moi-pholo- 

 gical sense, since there is no actual change 

 of condition ensuing with age, as has been 

 assumed by some authors, the fibrous layers 

 being always •originally deposited on the 

 primary wall in the form and pattern which 

 they ultimately possess. There appears to 

 be no j-eal opening of the s]^irals or break- 

 ing up into rings, in consequence of the 

 expansion of the primary wall to which 

 they are attached, 



