THE NERVOUS SYSTEM OF THE DOG 



725 



weight is 30 g. or even less, while in very large dogs it may exceed 150 g. It cor- 

 responds much more closely with the cranium in size and general form than in the 

 animals previously described; this is specially true of the small breeds, in which the 

 bony crests and frontal sinuses are little developed. 



The medulla is Isroad and thick. Its ventral surface is strongly convex from 

 side to side. The pyramids are large and prominent and are limited by distinct 

 median and lateral grooves. The olivary eminence is a well-defined oval elevation 

 situated between the pyramid and the superficial origin of the hypoglossal nerve. 

 The external arcuate fibers form a wide band which crosses the lateral surface 

 obliquely upward and forward, and obscures the groove which limits the restiform 



Fid 



CAo 



Fig. 541. — Part of Cross-section of Brain of Dog, Passing through Ganglion Habenul^e. 

 Al, Ansa leaticularis of thalamic radiation; C a d, dorsal end of hippocampus; C a v, ventral end of same; 

 C c, corpus callosum; C g m, corpus geniculatum mediale; C m, corpus mammillare; F i d, fimbria; F i v, ventral 

 part of fimbria; G h, ganglion habenulae; M B, fasciculus retroflexus; P p, pes pedunculi; T h, thalamus; T o, 

 tractus opticus; T t, habenula; V A, fasciculus thalamo-mammillaris; V F, fornix ventralis; //, lateral ven- 

 tricle; ///, third ventricle. (After Probst.) 



body externally. The cuneate tubercle is distinct. The rhomboid fossa is deep 

 and narrow. 



The pons is relatively small. The corpus trapezoideum is very wide and is 

 divided by the pyramids into two lateral parts. The cerebellum is very Inroad, but 

 is low and also compressed from before backward. Rather more than half of it is 

 overlapped by the cerebral hemispheres. The anterior surface is accurately 

 adapted to the concave tentorial surfaces of the hemispheres and to the posterior 

 corpora quadrigemina and their commissure. The posterior surface is almost 

 vertical and is convex centrally and flattened laterally. The vermis is prominent 

 and in general well defined, although it is connected in its middle part with the 

 hemispheres. The latter are three-sided and consist of four lobules. The anterior 

 peduncles are very short. 



The posterior corpora quadrigemina are large, very wide apart, and prominent, 

 and are connected by a curved commissure. The internal geniculate body is large. 



