348 



DISSECTION OF THE THORAX. 



forms a flattened disk between each pair of the vertebra, and its width is 

 determined by the size of the bones. It is connected in front and behind 

 with the anterior and posterior common ligaments, and on the sides with 

 the stellate ligament of the head of the rib. 



By means of the dissection before made, the intervertebral substance 

 may be observed to consist of two distinct parts ; an external, h'rm and 

 laminar, and an internal, soft and elastic (fig. 112). 



The outer laminar part (fig. 113, a) forms more than half of the disk, 

 and is composed of pieces of fibro-cartilage with layers of fibrous tissue. 

 All the strata are arranged one within another, like the scales of an onion, 

 and are connected by their edges to the bodies of the vertebrae ; but all 

 have not a vertical direction, for whilst the outer pieces are straight, the 

 inner are bent with the convexity outwards (fig. 112) (Humphry). The 

 laminae do not form complete rings, but those composed of fibrous tissue 

 reach farther than those of fibro-cartilage : the free ends of both layers 

 overlap. 



Each layer is constructed of oblique fibres ; and the fibres of one layer 

 are directed across those of another like the parts of the letter X (fig. 111). 

 This disposition of the fibres will be best seen on the disks between the 

 lumbar vertebrae ; and it may be rendered evident by dissecting layer after 

 layer. 



Fig. 111. 



Fig. 112. 



INTERVERTEBRAL SUBSTANCE IN THK LUM- VERTICAL SECTION OF THE INTERVERTEBRAL SUB- 

 BAR REGION, with its laminae displayed. STANCE, to show the direction of the layers. 

 a. Superficial, and b, deeper layer, the fibres a. Circumferential layers bent outwards, 

 in each taking a different direction. b. Central pulpy part. 



The central or pulpy portion of the disk (fig. 113, I) is very soft and 

 elastic, and projects when two vertebrae and the interposed mass are sawn 

 through. Placed nearer the back than the front of the disk, it is more 

 marked in the loins and neck than in the dorsal region. It has a yellow- 

 ish color, and is deficient in the stratiform arrangement so conspicuous 

 at the circumference. 



Towards the confines of the two portions of the intervertebral subslance, 

 there is an alternating arrangement of fibrous tissue and fibro-cartilage, 

 though the former is gradually diminishing; but towards the centre a loose 

 fibro-cartilaginous material with large cells preponderates. 



