THE WEIGHT OF THE BRAIN AND SPINAL CORD. 89 



of the portion about which our information is most 

 meagre, it may be said that we know very little concern- 

 ing the physical properties of the spinal cord, a lack for 

 which the comparative difficulty of obtaining the cord 

 for examination is to a large extent responsible. It is 

 always weighed after the removal of the dura, but with 

 the pia adherent. It should be added, however, that the 

 weight of the cord also varies with the manner in which 

 the nerves are separated from it. Since the nerves 

 arising along the cord may pass for several inches 

 within the canal before going through the vertebral 

 foramina, it becomes possible to separate them either at 

 the point where they arise from the cord or at the point 

 where they enter the foramina. Having a considerable 

 mass they add materially to the weight of the cord, 

 which therefore varies accordingly as it is weighed with 

 or without them. Occasionally the cord is farther 

 divided into the portions designated as the cervical, 

 thoracic, and lumbar, and these weighed separately. 



At the base of the skull the portion of the central 

 nervous system within the cranium contracts to form 

 the bulb, which is directly continuous with the spinal 

 cord, and one of the first difficulties is to determine the 

 proper point of separation between these two. The 

 mere acceptance of the mass within the cranial cavity as 

 the encephalon does not satisfy the conditions, because 

 both the brain and the spinal cord are capable of some 

 longitudinal movement, and as a result the amount 

 which may be included within the cranial cavity is 

 slightly variable. It has consequently become the custom 

 to select as a fixed point the caudal end of that eleva- 

 tion on the ventral surface of the bulb, known as the 

 decussation of the pyramids, and to make the section at 

 this level. The mass above this section is the ence- 

 phalon. This also establishes the caudal boundary of 



