288 



NORMAL HISTOLOGY. 



while other tracts contain mostly small ones. With reservation, it 

 may be assumed that motor fibres are generally the largest (15 

 20 ft): hence the nerves issuing from the anterior cornua contain 

 principally fibres of large size ; the posterior sensory nerves and 

 the sensory tracts, on the contrary, contain chiefly small fibres, 

 although a number of fibres of large diameter are usually present. 

 The largest fibres occur within the direct and crossed pyramidal 

 tracts ; the smallest, within the column of Goll. 



The white commissure in man forms a continuous nervous 

 lamella, .3-. 5 mm. in thickness, which separates the gray com- 

 missure from the bottom of the anterior median fissure ; in many 

 animals the white commissure is incomplete, being represented by 

 isolated commissural bundles found only at certain levels. 



The gray matter, while presenting the general H-form through- 

 out the cord, differs in the details of its arrangement in the several 



regions. The gray mat- 



FIG. 321. ter shares in the in- 



creased size which char- 

 acterizes the cervical and 

 lumbar enlargements, its 

 amount being absolutely 

 as well as relatively 

 greatest in the lumbar 

 region. 



Typical cervical sec- 

 tions are distinguished 

 by their large size, great 

 transverse diameter, and 

 large H of gray matter, 

 the anterior cornua of 

 which are robust and 

 broad, while the poste- 

 rior horns are slender. 

 Sections from the tho- 

 racic region are smaller 



than those from either above or below, and present an almost circular 

 outline ; the gray matter possesses crescents only slightly curved, with 

 slender horns both in front and behind. Cross-sections of the 

 lumbar cord are recognized by being broad, having a deep anterior 

 fissure, and possessing a large, thick H which greatly encroaches 

 upon the white matter. The latter diminishes relatively, as well as 

 absolutely, on reaching the conus medullaris, where it is reduced 

 to a mere shell. 



The gray matter of the halves is united by the gray commissure, 



Section of spinal cord from thoracic region of child : V, D, 

 ventral (anterior) and dorsal (posterior) median fissures; C, 

 column of Clarke. 



