356 PHYSIOLOGY 



gives off a series of nerve-roots, which are arranged in thirty-one pairs 

 and are distributed symmetrically to the two sides of the body. Each 

 nerve arises by two roots, an anterior and a posterior, the anterior 

 being composed of a series of rootlets spread over a considerable 

 area of the cord, while the posterior roots arise as a compact bundle 

 from a groove on the postero-lateral aspect of the cord. The posterior 

 nerve-roots pass through a ganglion and join the anterior roots in the 

 intervertebral foramina to form the mixed nerve. On section the cord 

 is seen to consist of a core of grey matter surrounded on all sides by 

 white matter. The white matter is made up of medullated nerve fibres 

 which are devoid of a neurilemma, and run within tunnels or tubes in 

 the supporting neuroglia. The grey matter has roughly the form of 

 a letter H, and consists, in cross-section, of a comma-shaped mass 

 on each side of the cord, joined across the middle line by a band of grey 

 matter. On the anterior aspect of the cord is a furrow, the anterior 

 fissure, which contains a process of the enveloping membrane of the 

 cord, the pia mater, and is limited at its bottom by a band of white 

 matter, the anterior white commissure, which unites the anterior 

 columns of white matter. 



On the hinder aspect of the cord is another fissure, the posterior 

 fissure, which is very narrow and is built up chiefly by neuroglia. A 

 third fissure at the point of origin of the posterior nerve-roots serves to 

 divide the white matter of the cord into an antero- lateral column and a 

 posterior column, and the former is imperfectly separated by the 

 spread-out anterior rootlets into anterior and lateral columns. The 

 cord in cross-section (Fig. 155) is circular in the dorsal region and oval in 

 the cervical and lumbar regions. It presents two marked enlargements, 

 namely, the cervical enlargement, corresponding to the outflow of the 

 nerves going to the upper limb, and the lumbo-sacral enlargement, 

 which gives off the nerves to the lower limb. In the sacral region it 

 rapidly tapers off to a blunt point. In the centre of the band of grey 

 matter, connecting the two masses on each side of the middle line, is the 

 central canal of the cord, the remains of the primitive neural canal of 

 the embryo. The grey matter in front of it is called the anterior grey 

 commissure, that behind the posterior grey commissure. The comma- 

 shaped mass of grey matter on each side of the cord presents in front 

 the broad anterior cornu, and behind the narrower posterior cornu, 

 which extends up to the postero-lateral groove in the line of emergence 

 of the posterior roots. In the dorsal region of the cord the grey matter 

 projects into the lateral column of white matter to form the lateral horn. 

 The grey matter consists of a supporting tissue of neuroglia in which 

 are embedded nerve-cells and their processes and the endings of nerve 

 fibres. The neuroglia is formed of a thick felt- work of fibres with here 

 and there nuclei applied to the fibres. Occasionally we may meet 



