84 ANATOMY OF THE CENTEAL NEETOUS SYSTEM. 



dom no such thing as a brain which is throughout of a higher or lower 

 degree of development. Now here, now there, certain parts of the brain 

 are more developed, and this development depends in no way upon the 

 position of the animal in the phylogenetic series, but solely upon require- 

 ments: i.e., upon the somatic characteristics which the animal has acquired 

 in this or that region. 



You will find that the bony fishes possess an exceedingly simple brain, 

 which is not at all to be compared with that of man. But thesS animals 

 possess not only much larger terminations for the optic nerves than does 

 any mammal, but the nerve-nuclei in its medulla have such a development 

 and such complicated relations that the same structures in reptiles, birds, 

 and mammals seem, in comparison, very small and simple. 



Thus the anatomical picture of the medullar nuclei of the cranial 

 nerves varies much in the animal kingdom. I will, therefore, try to present 

 a few facts which are common to all. 



Let us divide the cranial nerves in question into two groups: a posterior 

 group, including the Hypoglossus, the Accessorius, the Vagus, and the 

 Glosso-pharyngeus; and an anterior group, including the Facialis, the Acus- 

 ticus, and the Trigeminus. 



If one views the cranial nerves only from the stand-point of the rela- 

 tions of their deep origin, one finds, throughout the whole animal kingdom, 

 an astonishing similarity. For example, the terminal nuclei for the cranial 

 nerves are, in fishes and mammals, located the same. They vary consider- 

 ably, however, in the way in which the roots leave the central system. Sub- 

 sequent to the origin of the nerves there takes place the most varying com- 

 binations of the root-fibers; so that the certain tracing of them to the 

 surface of the brain is, in the lower vertebrates, a task on which comparative 

 morphology is still at work. For example, the Facialis is sometimes so 

 mixed with the fibers of the Trigeminus that it is only in the distribution 

 at the periphery that it may be dift'erentiated from that nerve. 



The ventral columns of large ganglion-cells have been located as the 

 origin of the motor nerves of the spinal cord. This cell-column may be 

 demonstrated from the sacral region to the medulla. It consists of a series 

 of nerve-nuclei between which lie commissural cells. It is advisable to 

 separate this column into two series: a more ventral one (Anterior-horn zone 

 of His) and a more lateral one ("Lateral-horn zone"). From the first arise 

 the Hypoglossus and all of the anterior roots of the spinal cord for the 

 trunk-musculature. From the latter arise principally fibers which are de- 

 voted to the motor innervation of the viscera, only in the medulla. These 

 lateral-horn fibers become separated from the anterior-horn fibers and leave 

 the medulla as motor fibers of the A^agus and Accessorius. Farther down 

 the spinal cord they leave with other fibers of the anterior roots. According 



