528 TEXT-BOOK OF PHYSIOLOGY 



The efferent nerves taken collectively are also distributed to skeletal 

 muscles, glands, blood-vessels and viscera. 



The afferent nerves taken collectively are distributed to the skin, mucous 

 membrane and to specialized sense organs eye, ear, nose, tongue and skin. 

 Some of the afferent nerves contain efferent fibers which are distributed to 

 glands, blood-vessels and viscera. 



The origin and distribution of both efferent and afferent encephalic nerves 

 and the phenomena that follow their stimulation and divisions, and the func- 

 tions attributed to them will be fully considered in a subsequent chapter, j 



THE SPINAL CORD 



The narrow elongated portion of the central nerve system contained 

 within the spinal canal is named, from its situation and appearance, the 

 spinal cord. It is cylindric in shape though presenting an enlargement in 

 both the lower 'cervical and lower lumbar regions corresponding to the origins 

 of the .nerves distributed to the upper and lower extremities. The cord 

 varies in lengthTfFom 40 to 45 cm., measures 12 mm. in diameter, weighs 

 42 gms., and extends from the atlas to the second lumbar vertebra, beyond 

 which it is continued as a narrow thread, ihefilum terminate. It is divided 

 by the anterior and posterior longitudinal fissures into halves, and is there- 

 fore bilaterally symmetric. A transverse section of the cord shows that 

 it is composed of both white and gray matter, the former covering the surface, 

 the latter- occupying the center. 



The Segmentation of the Spinal Cord. For the elucidation of many 

 problems connected with the physiologic actions of the spinal cord, as well as 

 of the symptoms which follow its pathologic impairment, it will be found 

 helpful to consider the cord as consisting physiologically of a series of segments 

 placed one above the other, the number of segments corresponding to 

 the number of spinal nerves. Each spinal segment would therefore comprise 

 that portion of the cord to which is attached a pair of spinal nerves. The 

 nerve-cells in each segment are in histologic and physiologic relation with 

 definite areas of the body, embracing muscles, glands, blood-vessels, 

 skin, etc. 



If the exact distribution of the nerves of any segment were known, 

 its function could be readily stated. By virtue of this segmentation it 

 becomes possible for each segment to act independently of or in cooperation 

 with other segments near or remote, with which they are associated by the 

 intrinsic or associative cells and their axons; and by the same cooperative 

 action the spinal cord itself is enabled to act as a unit. 



The Structure of the Gray Matter. The gray matter is arranged in 

 the form of two crescents, united in the median line by a transverse band 01 

 commissure forming a figure resembling the letter H. Though varying 

 in shape in different regions of the cord, the gray matter in all situations 

 presents on either side an anterior or ventral and a posterior or dorsal horn. 

 Between the two horns there is a portion termed the intermediate gray sub- 

 stance. The commissure presents in its center a narrow canal which extends 

 throughout the entire length of the cord. This canal is lined by cylindric 

 epithelium and surrounded by gelatinous material. (Fig. 227.) 



The anterior horn is short and broad and entirely surrounded by white 

 matter. The posterior horn is narrow and elongated and extends quite 



