1334 



A MANUAL OF ANATOMY 



emerge through the intervertebral foramina, which along with 

 the offsets of the anterior and posterior intraspinal veins, form 

 plexuses around the spinal nerves. From these plexuses the blood 

 is conveyed on either side into the vertebral plexus and deep 

 cervical vein in the neck, and into the intercostal and lumbar veins 

 in the corresponding regions. 



Lymphaties. — There are no l5aiiphatic vessels in the spinal cord. 

 Their place is taken by spaces in the outer coat of the arteries, 

 called perivascular spaces, which are in communication with the 

 subajachnoid space. 



Development of the Spinal Cord. 



The spinal cord is developed from the wall of the neural tube, and the 

 neural tube is developed from the neural plate. This plate, situated in the 

 anterior part of the embryonic area, consists of ectodermic cells, and it presents 



Posterior Nerve-Root 



Spongioblasts 



Neuroblasts 



Anterior Nerve-Root 



Fk5. 557. — Section of the Spinal Cord of a Human Embryo of 

 Four Weeks (His). 



a median longitudinal groove, called the neural groove. The lateral margins 

 of the plate become elevated into two ridges, the neural folds, and these by 

 their approximation and fusion give rise to the neural tube or canal. At 

 either end the neural tube presents an opening — the anterior and posterior 

 neuropore, but these openings soon become closed. 



The ectodermic cells of the wall of the tube undergo proliferation, the wall 

 becomes thickened, and it consists of two kinds of cells — namely, (i) sus- 

 tentacular or supporting cells, and (2) nerve-cells proper. The former con- 

 stitute the ependyma and neuroglia of the spinal cord, and the latter give 

 rise to the grey and white matter. The cells are loosely arranged, and form 

 what is known as the myelospongium. The cells of this myelospongium 

 become condensed internally and externally, and these condensed layers 

 form the internal and external limiting membranes. The wall of the neural 



