THORAX AND ABDOMEN OF THE HORSE 61 



branches that are concerned in the formation of limb-plexuses are larger 

 than those not so concerned. 



Veins of the vertebral canal. — If the floor of the vertebral 

 canal be examined, a large venous trunk — really a sinus (sinus 

 vertebralis lougitudinalis) — will be found running along each border of 

 the dorsal longitudinal ligament. Transverse communications between 

 the two sinuses pass underneath the ligament, and into them open 

 veins from the interior of the bodies of the vertebrae. Veins from the 

 spinal cord (ven^e spinales) and its membranous coverings also open 

 into the sinuses. 



The sinuses are drained by effluents that pass out of the vertebral 

 canal by the intervertebral foramina and, in the thoracic, lumbar and 

 sacral regions, open into the intercostal and the lumbar and sacral 

 spinal veins. 



The articulations of the vertebral column (articulationes 

 columnfe vertebralis). — When two macerated neighbouring vertebrae 

 are placed in their proper position relative to each other, they come 

 into contact at three points — the bodies and the articular processes. 

 In the recent state, however, the bodies do not actually touch each 

 other because, interposed between them, there is a disc-shaped 

 intervertebral fibro-cartilage (fibrocartilago intervertebralis), thickest 

 in the cervical and coccygeal regions, and thinnest between the thoracic 

 vertebrae. The peripheral part of each fibro-cartilage contains dense 

 fibres arranged in a ring-like manner (annulus fibrosus), while the 

 centre of each cartilage, the remains of the embryonic notochord, is 

 softer and yellower (nucleus pulposus). 



Forming a further connective medium between the vertebral bodies 

 are two longitudinal ligaments. 



The ventral longitudinal ligament (ligamentum longitudinale 

 ventrale) is attached along the ventral surface of the vertebral bodies 

 from the eighth or ninth thoracic to the last lumbar. It ends by 

 expanding and merging into the periosteum of the sacrum. The 

 ligament widens slightly opposite each intervertebral fibro-cartilage, 

 with which it is intimately connected. 



The dorsal longitudinal ligament (ligamentum longitudinale 

 dorsale) extends along the floor of the vertebral canal from the dens of 

 the epistropheus to the sacrum. It is attached firmly to the dorsal 

 aspect of the vertebral bodies and the intervening fibro-cartilages. The 

 width of the ligament is not uniform. Opposite the intervertebral 

 fibro-cartilages it is broad; but opposite the middle of each vertebral 

 body, where the transverse communications between the two longi- 

 tudinal venous sinuses run beneath it, it is much narrower. 



