604 



long reaction system has been constructed. For this reason, it may be 

 stated that reflex action is a more primitive function than the type of 

 conduction seen in the higher animals. But, while the spinal cord of 

 the latter has lost much of its simple reflex character, it cannot be 

 denied that it still displays it in a clearly recognizable manner. Thus, 

 we have seen that this structure contains a series of centers for super- 

 ficial, deep and organic reflexes, and that the location of these centers 



roughly corresponds to the seats of these 

 actions, i.e., they are arranged in accord- 

 ance with a definite segmental pattern. 

 In addition, the succeeding discussion will 

 show that this segmentalism and dissocia- 

 tion of function has also entered into the 

 construction of the conducting paths. 



FIG. 295. THE MEMBRANES OF THE 



SPINAL CORD. 

 1. Dura mater. 2. Arachnoid. 



3. Posterior root of spinal nerve. 



4. Anterior root of spinal nerve. 5. 

 Ligamentum dentatum. 6. Linea 

 splendens. (After Ellis.) 



FIG. 296. TRANSVERSE SECTION 

 THROUGH THE REGION OF THE FOURTH 

 CERVICAL VERTEBRA. 



V, Body of vertebra; B, verte- 

 bral blood-vessels; N, spinal nerve; 

 RC, ramus communicans; S, spinal 

 ganglion; A, subarachnoidal space 

 investing spinal cord. 



The spinal cord of man appears as a cylindrical structure which extends into 

 the vertebral canal for a distance of 40-45 cm., i.e., to the level of the second or 

 third lumbar vertebra. Beyond this point it continues as a narrow thread, called 

 the filum terminale. It measures 12 mm. in diameter and weighs 42 grams. 

 From it arise thirty-one pairs of nerves, in serial order so that each pair corre- 

 sponds to a vertebra and innervates symmetrical areas upon the two sides of the 

 body. The spinal nerves are mixed nerves, i.e., they consist of afferent and efferent 

 fibers connecting central parts with their respective receptors and effectors. It is 

 to be noted, however, that they do not arise as such directly from the cord, but 

 originate as two compact bundles, one of which lies in close relation with the an- 

 terior and the other with the posterior aspect of this structure. The former 

 constitute the anterior (ventral) root and are efferent in their nature, while 

 the latter form the posterior (dorsal) root and conduct only in an afferent direction. 

 These two groups of fibers are joined in the intervertebral foramina, their point 

 of union being roughly marked by a ganglion composed of the cell-bodies belonging 

 to the sensory fibers of the posterior root. The nerves which are distributed to 



