152 



THE ARTICULATIONS OF THE TRUNK AND HEAD. 



most of these layers consist of ordinary fibrous tissue, but the deeper and more numerous 

 laminae consist of white fibre-cartilage. The central part of the disc is a pulpy and 

 elastic material which, when the pressure confining it is taken off by cutting 

 through the intervertebral substance, rises up so as to assume a conical form. It is- 



Fig. 174. A LUMBAR VERTEBRA, SEEN FROM ABOVE, WITH PART* 

 OP THE INTERVERTEBRAL DISC ADHERING TO THE BODY, 



(R. Quain.) 



1, 1, the fibrous laminae arranged concentrically 

 central soft substance. 



2, the 



then seen to be of a lobate structure, and, examined 

 under the microscope, exhibits a finely fibrous matrix, 

 imbedded in which are seen numerous cells which 

 are not of the nature of cartilage cells, but are 

 united together so as to form a reticular structure, 

 which is closer in the centre of the pulp than 

 towards the periphery. A thin cartilaginous layer 



covers the upper and lower surfaces of each vertebra and gives attachment to the 

 disc, but it is incomplete towards the circumference, where the epiphyses of the body 

 have been developed in it. 



It is generally admitted that the pulp of the intervertebral disc is in part at least a remains 

 of the chorda dorsalis, homologous, therefore, with those larger vestiges of the chorda 

 dorsalis which occupy the biconical cavities between the bodies of the vertebras in fishes, 

 According to Luschka, there is present in each disc a synovial cavity, and the lobes of the 

 pulp are synovial villi, similar to those which are to be found in diarthrodial joints, but of 

 larger size, and occupying the whole cavity ; and it is worthy of notice that in like manner 

 secondary cavities, developed within the chorda dorsalis, are found in the intervertebral 

 substance in many fishes. The same author also describes in the cervical region a synovial 

 joint with cartilage-covered surfaces, on each side, between the prominent lip of the upper 

 surface of the body of one vertebra and the corresponding portion of the under surface of the 

 body of the vertebra above. (Luschka, " Die Halbgelenke des menschlichen Korpers," Berlin, 

 1858.) 



The discs are thickest, both absolutely and relatively to the depth of the vertebral 

 bodies, in the lumbar region, and thinnest in the intervals from the third to the 



Fig. 175. SAGITTAL SECTION THROUGH TWO- 

 LUMBAR VERTEBRAE SHOWING THE ARRANGE- 

 MENT OP THE INTERVERTEBRAL DISC. 



(R. Quain.) | 



1, 2, the fibrous laminae ; 3, the central soft 

 substance ; the capsule of the joint between the 

 articular processes is also shown. 



seventh dorsal vertebrae. They form 

 together about a fourth of the length 

 of the moveable part of the column. 

 In the cervical and lumbar regions 

 they are thicker in front than behind, 

 and the curvature of these portions of the column is due principally to the form 

 of the discs. 



The auterior common ligament (fig. 184, p. 158) is a strong band placed 

 on the front of the bodies of the vertebrae, and reaches from the axis to the first 

 piece of the sacrum, becoming broader as it descends. It consists of longitudinal 

 fibres which are dense, firm, and well-marked. The superficial fibres extend from 

 a given vertebra to the fourth or fifth below it ; the fibres beneath these pass over 



