110 ANTHROPOLOGY. 



nervous system. This we shall consider under the heads of the spinal cord 

 . and of the brain or encephalon, the latter including the medulla oblongata, 

 the mesocephalum, the cerebellum, and the cerebrum. 



B. The Spinal Cord. 



Tlie spinal cord occupies a large portion of the spinal canal, terminating 

 inferiorly at a point, which in different subjects ranges between the last 

 dorsal and the second lumbar vertebra. Below this point, the sheath 

 formed by the dura mater contains that series of nerves known as the cauda 

 equina. The entire length in the adult is from sixteen to eighteen inches. 

 Its circumference measures twelve lines at the smallest, and eighteen at the 

 most voluminous part. Its weight is said to be from the y\ to the 2V of 

 that of the brain in the adult, and about the -^ P^i't in the new-born infant. 

 The actual weight of the spinal cord in the adult may be taken at a little 

 over one ounce. 



''J'he consistence of the medullary substance of the spinal cord, when fresh, 

 is much greater than that of the brain. The pia mater adheres very closely 

 to its surface, like neurilema to a nerve. It is penetrated both on the an- 

 terior and posterior aspect by fissures, each of which corresponds to the 

 median plane. They are separated from each other by a transverse bilami- 

 nate partition of white and gTcy matter, of which the grey layer is posterior. 

 This serves to connect the equal and symmetrical portions into which the 

 cord is divided by these fissures. The anterior of these fissures is deeper 

 than the posterior. The two taken together seem to indicate the existence 

 of two spinal cords, one for each side of the body, both presenting a perfect 

 resemblance in form and structure. 



On examining a transverse section of the spinal cord, we find that the 

 interior of each lateral portion is occupied by grey matter disposed in a 

 crescentic form, exactly similar in each, and connected by the grey com- 

 missure, a layer which extends between the two crescents, being attached 

 nearly to the central point in each. This commissure forms a vertical plane 

 • of grey matter throughout the whole of the cord. The concavity of the 

 lateral crescent is directed outwards. Their anterior extremities are thick, 

 and separated from the surface of the cord by a considerable layer of white 

 fibrous matter. The posterior extremities of the crescents are prolonged 

 backwards and outwards in the form of a narrow horn, which reaches the 

 surface of the cord. These posterior horns constitute on each side a natural 

 boundary, between the two columns of which each lateral half of the cord 

 consists. All in front of the posterior horns is called the antero-lateral 

 column, comprehending the white matter forming the sides and front of the 

 semi-cord, limited anteriorly by the anterior fissure, and posteriorly by the 

 posterior roots of the nerves. The posterior column is situated behind the 

 posterior horn of grc}^ matter, and is separated from its fellow on the oppo- 

 .site side by the posterior fissure. 



There are these four columns in the spinal cord, two antero-lateral and 

 two posterior. The former constitute by far the larger proportion of the 

 white substance of the cord, and they envelope the anterior obtuse portion 

 816 



