THE NERVOUS SYSTEM 427 



the minute structure rather than in the external form, in which 

 latter respect it is singularly simple and uniform throughout all 

 vertebrates. Its sides and floor, which alone come into consid- 

 eration as a nervous organ, together with the crura cerebri and 

 the pons, form the central system of commissures for the entire 

 nervous system, receiving the fibers from all other parts and 

 forming the necessary connections of these with one another 

 and zvith the spinal cord. That these connections become 

 vastly more complex in the higher than in the lower vertebrates 

 is evidenced both by the gradual growth of the various parts 

 of the brain in size and complexity, and by the results as seen 

 in the behavior of living animals. A rhomboidal area which 

 includes the greater part of the roof of this part remains mem- 

 branous and forms an important chorioid plexus, that of the 

 fourth ventricle (tcenia ventriculi quarti). As this thin place 

 and its subjacent rhomboid cavity (fossa rhomboidalis) are 

 extremely conspicuous objects in all embryos, this portion 

 of the brain is often conveniently termed the rhomben- 

 cephalon. 



Morphologically the medulla is the anterior continuation of 

 the spinal cord, and the nerves that proceed from it resemble 

 the spinal nerves more than do those which arise farther for- 

 ward. In fact the line of division between medulla and cord 

 is an artificial one, the first being considered as coterminous 

 with the skull in all cases. Similarly those nerves in that 

 region which obtain their exit through a foramen in the skull 

 are termed cranial and are accorded to the medulla. The ar- 

 tificial character of this distinction involves confusion at one 

 point at least, namely, the varying limits of the skull between 

 amphibians and reptiles due to the absorption of a vertebra. 

 (See Chap. V.) In this way the hypoglossal nerve (Xllth), 

 a spinal nerve in amphibians, becomes added to the list of 

 cranial nerves in the Amniota, although this case involves 

 rather more than the simple addition of a single pair of spinal 

 nerves, and is still a somewhat obscure point. 



Beyond the medulla the neural tube becomes the spinal cord, 

 which, although it often shows some little regional differentia- 

 tion, is far more conservative than the anterior portion and 



