NERVOUS SYSTEM. 
I. THE CENTRAL NERVOUS SYSTEM. 
1. 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 vertebree. 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 vertebre (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, c), along 
the ventral median line; into this the pia mater dips. The 
posterior median sulcus (Fig. 133, ¢; Fig. 134, a) isa 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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