226 



ELEMENTS OF MAMMALIAN ANATOMY 



the ventricles of the brain and the aqueous humor of the eye. 

 It seems to be supplied for the most part by the choroid plexus 

 of the ventricles although other sources have been suggested. 

 The cord presents two enlargements, the cervical, whence 

 issue the nerves of the forelimbs, and the lumbar, giving origin 



Fig. 1 10. — Photomicrograph of Cross-section of Human Spinal Cord. 



a, Arachnoid membrane; an, ventral side in which appears the deep anterior 

 fissure; d, dura mater; n, one of several nerve bundles; the tiny white spot in the 

 center of the dark gray matter is the canalis centralis; p, pia mater. X 5. 



to the nerves of the posterior limbs. Two deep fissures, the 

 ventral (anterior) median and the dorsal (posterior) median, 

 penetrate about one-third through the cord, incompletely divid- 

 ing it into halves longitudinally. The pia mater is prolonged 

 into the ventral fissure, but not into the dorsal fissure. Slightly 

 laterad of the ventral median fissure issue the ventral roots 

 of the spinal nerves, and at about the same distance from the 

 dorsal median fissure are the dorsal roots of the spinal nerves. 

 These two roots unite about one centimeter from their origin 



