185 



LESSON XXXVI. 



STRUCTURE OF THE SPINAL CORD. 



1. SECTIONS of the spinal cord from the cervical, dorsal, and lumhar regions. 



[The spinal cord is hardened by being immersed for about a month in 

 bichromate of ammonia solution (2 per cent.) ; the sections are made with a 

 freezing microtome and are stained with aniline blue black. 1 ] 



Notice the relative extent of the grey as compared with the white matter 

 in the different regions of the cord. In the white matter the stained dots 

 are sections of the axis-cylinders of the longitudinally disposed medullated 

 nerve-fibres ; the tissue between the nerve-fibres is neuroglia. In the grey 

 matter the branched nerve-cells are conspicuous, lying in a reticular sub- 

 stance which is formed of neuroglia together with an interlacement of nerve- 

 fibres and cell-processes. 



Sketch a section from each region under a low power. Sketch also a 

 small portion of the white substance, two or three nerve -cells, and the central 

 canal with its lining epithelium and surrounding neuroglia under the high 

 power. 



Measure the diameter of some of the nerve -fibres in the anterior columns, 

 in the lateral columns, and in the posterior columns. 



The spinal cord is composed of grey matter in the centre and of 

 white matter externally. It is closely invested by a layer of connective 

 tissue containing numerous blood-vessels (pia mater), and less closely 

 by two other membranes. One of these is an areolar membrane, re- 

 sembling a serous membrane in general structure, but non-vascular and 

 more delicate in texture (arachnoid). The other, which lines the 

 vertebral canal, is a strong fibrous membrane known as the dura mater. 

 At the middle of the anterior and posterior surfaces the pia mater dips 

 into the substance of the cord in the anterior and posterior median fis- 

 sures, so as to divide it almost completely into two lateral halves. These 

 are, however, united by an isthmus or bridge, which is composed 

 anteriorly of transversely crossing white fibres (white commissure), 

 posteriorly of grey matter (grey commissure), in the middle of which is 

 a minute canal lined by ciliated epithelium (central canal, fig. 221, e). 



Each lateral half of the spinal cord contains a crescent of grey 

 matter, which is joined to the corresponding crescent of the opposite 

 side by the grey commissure. Of the two cornua of the crescent the 



1 See Appendix. 



