NERVOUS SYSTEM 837 



posterior end, but when legs have developed with an increase of muscu- 

 lature, the spinal cord becomes enlarged in the regions where the nerves 

 for the limbs are given off. Casts of the spinal canal in certain fossil 

 reptiles indicate that there was an accumulation of nervous matter near 

 the hind legs which exceeded the brain in size. 



The nerves leave the spinal cord at nearly right angles to its axis 

 when development begins. Then there occurs an inequality in growth, 

 the body increasing more in length than does the cord. As a result the 

 more caudal nerves pursue a very oblique course, and in the hinder part 

 of the spinal canal of the higher vertebrates they form a bundle of 

 parallel nerves, the cauda equina (horse-tail). Often, too, another result 

 of the unequal growth may be the drawing out of the hinder end of the 

 cord into a slender, non-nervous thread, the filum terminale (Fig. 17). 



Flexures (Fig. "288). 



In the early stages of development, it will be remembered, the head 

 end of the developing spinal cord bends forward at almost right angles 

 to the main axis and this first bend is called the primary flexure. The 

 second bending occurs at the most caudal end of the medulla oblongata 

 and is called the nuchal flexure ; it bends in the same direction as does 

 the first or primary. The third bend is at a level with the cerebellum and 

 is known as the pontal flexure; it bends in the opposite direction of the 

 other two, thus drawing back the fore part of the entire brain to lie on 

 top of the more rearward portion. 



The three flexures just mentioned remain throughout adult life in 

 all mammals, but even where one or more of the flexures appear in the 

 embryonic state in vertebrates lower than mammals, it is seldom that 

 more than one or two remain. In reptiles and birds the nuchal and 

 pontal flexures are weakly developed and entirely obliterated in the 

 adult. 



Neuromeres (Fig 278). 



There have been many interesting theories advanced in times past 

 as to whether or not skull and brain portion of animals were merely a 

 continued segmented portion of the spinal column and cord. There has 

 never been any satisfactory solving of the matter, however. This much 

 we know : that during its development the brain does show some traces 

 of segmentation in a linear arrangement. These segments are called 

 neuromeres, of which eight are well defined. Five lie in front of the ear, 

 one corresponds to the ear in position, and two lie behind the ear. 



It is from the first of these segments (though some insist there are 

 two here) that the fore-brain arises, as well as the parts which in turn 

 arise from it. The second becomes the mid-brain. The third lies in the 

 region of the cerebellum. The fourth and fifth lie in the region of the 



