414 



STUDIES IN ADVANCED PHYSIOLOGY. 



shows, however, two enlargements along its course, one in 

 the cervical region called the cervical enlargement, and a 

 second at the beginning of the lumbar region called the 

 lumbar enlargement. It weighs from one and one-half to 

 two ounces. 



The spinal cord gradually shades off into the brain, and 

 the point at which one is said to cease and the other to be- 

 gin is quite arbitrary. There is no 

 sudden transition from one to the other. 

 If a cross-section be made, the cord is 

 seen to consist of two halves, this bi- 

 lateral arrangement being caused by 

 two deep grooves or fissures running 

 longitudinally along the cord and al- 

 most separating it into a right and left 

 lobe. The division, however, is not 

 complete, the anterior and the posterior 

 fissures not meeting, but the two hemi- 

 spheres being connected near the mid- 

 dle by a commissure consisting partly 

 of gray matter, and anterior to this of 

 white matter. The posterior fissure 

 reaches down to the gray matter, and 

 the anterior fissure to the white com- 

 missure just referred to. In the center 

 of the gray commissure is the cross- 

 section of a canal running lengthwise 

 through the spinal cord and connected 

 in the brain with the ventricles. This 

 is called the central canal, or canal is 

 centralis. It probably has no specific 

 physiological function, but its presence is easily explained 

 by reference to the embryonic development of the spinal 

 cord and brain. 



A cross-section of the cord does not show a uniform ap- 

 pearance, but shows a grouping into two tissues, so dis- 

 posed that the central gray substance is arranged somewhat 



Fig. 137. CROSS-SECTION OF 



HUMAN SPINAL CORD, 

 TWICB NATURAL SIZE. 



A, cervical region; B, dor- 

 sal region ; C, lumbar region ; 

 a, anterior root; p, posterior 

 root. 



