316 PHYSIOLOGY 



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 inter vertebral 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. 154) 

 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 cells provided with a very large 

 number of branches and representing the cells from which all the fibres of the 

 neuroglia have been derived. The neuroglia is present in specially large 

 amount in two situations, namely, immediately around the central canal 

 and as a capsule to the enlargement of the posterior cornu, known as the 

 head or caput cornu posterioris. In this latter situation the neuroglia con- 

 tains a large number of small richly branched nerve- cells and is spoken of as 

 the substantia gelatinosa of Rolando. The nerve-cells are arranged in 



