THE NERVOUS SYSTEM. 317 



tongue styloglossus, hyoglossus, genio-hyoid, genio-hyoglossus, 

 communicating with the pneumogastric, sympathetic, first and 

 second cervical, and gustatory nerves. Its branches are : 



Descendens hypoglossi, Muscular, 



Thyro-hyoid, Meningeal. 



MEMBRANES OF THE CORD. The membranes of the spinal 

 cord are three the dura mater, arachnoid, and pia mater. 



Dura mater is a loose fibrous sheath, continuous with the 

 dura of the brain, and from which it differs in not inclosing the 

 venous sinuses, not dipping into the fissures of the cord, and 

 not being adherent to the bony canal. From the latter it is 

 separated by the venous plexuses and some connective tissue. 



It extends the whole length of the canal, from the foramen 

 magnum (to which it is attached) to the top of the sacrum. 



Arachnoid is a delicate serous sac, continuous above with the 

 cerebral arachnoid, inclosing the pia mater, from which it is 

 separated by an interval the subaraclinoidean space. It is filled 

 with the cerebro-spinal fluid. The outer surface of the arachnoid 

 is in contact with the dura, the space between them being called 

 the subdural space. 



Pia mater is a fibrous membrane, closely adhering to the 

 cord and forming its neurilemma. Over the anterior median 

 fissure it is strengthened by a fibrous band the linea splendens 

 and laterally has the ligamentum denticulatum. It terminates 

 below the cord as the.filum terminate a slender filament. 



THE SPINAL CORD. 



The spinal cord (medulla spinalis) is the elongated portion 

 of the cerebro-spinal axis contained in the spinal canal. Its 

 length is about sixteen to eighteen inches, extending from the 

 medulla above to the lower border of the first lumbar vertebra 

 below, where it terminates in the cauda equina by a slender pro- 

 longation of gray substance, called the conus medutlaris. 



It presents two enlargements, the upper or cervical, extend- 

 ing from the third cervical to the second dorsal vertebra, and 

 the lower about the position of the second or third dorsal verte- 

 bra. It is divided into two lateral halves by the anterior ami 

 posterior median fissures, united in the centre by the commissure. 

 The lateral portions are again subdivided by the anlcro-lulend 

 and postcro-lateral fissures into the anterior lateral and posterior 

 lateral columns,, and posteriorly a narrow fissure separates the 



