396 VEINS OF THE SPINAL MARROW. 



This process from the dura mater becomes so much changed, 

 that it has been considered as cellular membrane. 



The pia mater and tunica arachnoidea seem also to invest, 

 not only the nerve in general, but the fibres of which it is com- 

 posed. On this account, probably, the nerves are larger after 

 passing through the dura mater, than they are when they leave 

 the brain and spinal marrow. 



The Arteries of the Spinal Marrow proceed from the head, 

 and, with several additions, continue downwards to the lumbar 

 vertebrae. 



There is, generally, one artery on the front surface of the 

 medulla, which is formed by the union of two branches, that 

 arise from the vertebral arteries within the cranium. This 

 artery proceeds downwards and communicates with those of 

 the neck, and with the intercostal arteries, by the intervertebral 

 foramina, so that it preserves its size. 



It terminates with the spinal marrow ; and the cauda equina 

 below it, is supplied by branches from the internal iliac, which 

 enter through the foramina of the sacrum. 



There are, generally, two arteries on the posterior surface of 

 the medulla spinalis, which also pass out from the cranium ; 

 arising from the vertebral arteries or inferior arteries of the 

 cerebellum ; they have a serpentine arrangement, and com- 

 municate with each other, and with the ramifications of the 

 anterior spinal artery. 



All of these arteries are dispersed upon the spinal marrow and 

 its membrane, and the parts immediately contiguous. 



The veins correspond with the ramifications of the arteries ; 

 but they are collected into two larger branches called the Sinus 

 Venosi ; which are situated exterior to the dura mater, on the 

 front and lateral sides of the spinal canal. They extend the 

 whole length of the canal, and entering the great occipital fora- 

 men, communicate with the lateral and occipital sinuses. 



