120 Mammalia. Nervous System. 



The Brain, or Encephalon, presents four primary segments 

 or divisions, named respectively from back to front. 1. 

 Epenceplialon. 2. Mesencephalon. 3. Prosencephalon, and 

 4. E-hinencephalon. 



1. The hindmost division, or Ep-encephahn, consists of the 

 Macromyelon (= ' medulla oblongata,' + the intracranial pro- 

 longation of the myelonal columns as far forwards as their 

 emergence from the Pons), and the Cerebellum, together with 

 its transverse Commissure, called * tuber annulare,' or Pons 

 Yarolii. From the Macromyelon the cerebral nerves, from 

 the fifth to the hypoglossal (or ninth) inclusive, arise. In 

 most Mammals below Man, a transverse section of the Me- 

 dulla oblongata, in the region of the first cervical nerve, pre- 

 sents the form of an ellipse, as opposed to that of a circle, as 

 in Man. The Olivary bodies do not always present internally 

 a * corpus dentatum,' or nucleus with plicated capsule of 

 white neurine : they are frequently crossed before reaching 

 the Pons by the trapezoid homologues of the arciform fibres 

 in Man. The Trapezium^ which is found in most Mammals, 

 except Man, consists of a quadrangular elevated layer of 

 transverse medullary fibres, which lies close behind the Pons, 

 near to the pyramidal bodies, and abuts against the origin of 

 the auditory and facial nerves. The middle lobe of the 

 Cerebellum is the most constant part in the Vertebrate series, 

 but by no means always holds a large proportional size, as 

 compared with the lateral lobes. The addition of Lateral lobes 

 to the Cerebellum is characteristic of the Mammalian Class. An 

 * appendicular lobe,' a subspherical lateral process or append- 

 age to the hemispheres of the Cerebellum, is sometimes found. 

 To the * flocculus ' the origin of the acoustic nerve can be 

 traced; it is generally associated with large external ears, 

 and a well-developed auditory organ. In Mammals the ven- 

 tricle of the Cerebellum, which is persistent in Fish, Heptiles 

 and Birds, is obliterated. The Pons is found in all Mammals, 



