NERVOUS SYSTEM. 



I. THE CENTRAL NERVOUS SYSTEM. 



I. The Spinal Cord. Medulla spinalis (Figs. 133-136). 

 — The spinal cord is that portion of the nervous system which 

 occupies the vertebral canal ; it is continuous craniad with the 

 brain. It has the form of a somewhat flattened cylinder and 

 extends from the foramen magnum into the caudal region. It 

 diminishes in diameter after entering the sacral region. 



The cord has a cervical and a lumbar enlargement, the 

 former (Fig. 133) marking the origin of the nerves which pass 

 to the fore limb, and the latter (Fig. 136) the origin of those 

 which pass to the hind limb. 



The cervical enlargement (Fig. 133) lies in that part of the 

 vertebral canal bounded by the fourth to seventh cervical and 

 first thoracic vertebrae. Caudad of the first thoracic vertebra 

 the cord continues of nearly uniform diameter to the lumbar 

 enlargement (Fig. 136) which stretches from the third to the 

 seventh lumbar vertebrae (inclusive). Caudad of the seventh 

 lumbar vertebra it diminishes uniformly in diameter and ends 

 in a slender cord, the filum terminale (Fig. 136, C), which 

 may be traced into the caudal region. 



The surface of the cord is marked by a number of longi- 

 tudinal grooves or sulci, and fissures. The most prominent 

 of these is the anterior median fissure (Fig. 134, r), along 

 the ventral median line; into this the pia mater dips. The 

 posterior median sulcus (Fig. 133, e; Fig. 134, a) is a shallow 

 furrow along the dorsal median line. The anterior fissure and 

 the posterior sulcus thus divide the cord into lateral halves. 



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