THE NERVOUS SYSTEM OF THE DOG 847 



filaments which join the vagus near the ganglion nodosum. The cervical trunk is 

 short and relatively larger than in the ox; it is enclosed in a common sheath with 

 the vagus in the neck and separates from that nerve to join the middle cervical 

 ganglion at the thoracic inlet. 



THE NERVOUS SYSTEM OF THE DOG 



The spinal cord is almost circular in cross-section except at the well-marked 

 cervical and lumbar enlargements, where it is compressed dorso-ventrally. The 

 conus medullaris lies over the junction of the sixth and seventh lumbar vertebrae. 

 The length of the cord of a rather large dog was found to be about fifteen inches 

 (ca. 38 cm.) ; of this, the cervical part was about four and a half inches (ca. 11 cm.), 

 the thoracic a little less than seven inches (17.4 cm.), the Jumbar a little less than 

 three inches (ca. 7 cm.), and the sacro-coccygeal about an inch (ca. 2.6 cm.) 

 (Flatau-Jacobson) . 



The brain weighs about two to two and a half ounces (ca. 60-70 gm.) in dogs 

 of medium size, but there is, of course, a wide range of weight in the different breeds. 

 Thus in small terriers, the weight is about an ounce (ca. 30 gin.) or even less, while 

 in very large dogs it may exceed five ounces (ca. 150 gm.). It corresponds much 

 more closely with the external form of the cranium in size and general form than in 

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

 the bony crests and frontal sinuses are little developed. 



The meduUa oblongata is broad and thick. Its ventral surface is strongly 

 convex from side to side. The pyramids are large and prominent and are limited 

 by a distinct median fissure and lateral grooves. The median fissure ends at a 

 small depression, the foramen caecum, just behind the pons. 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 facial tubercle and the groove which limits the restiform body laterally. The 

 cuneate tubercle is distinct. The rhomboid fossa is deep and narrow. The 

 fourth ventricle" communicates with the subarachnoid space on each side through 

 a lateral aperture. 



The pons is relatively small, and in correlation with this fact the corpus trape- 

 zoideum is very wide; it is divided by the pyramids into two lateral parts. The 

 cerebellum is very broad, but is low and also compressed from before backvfard. 

 Rather more than half of it is overlapped by the cerebral hemispheres. The an- 

 terior surface is accurately adapted to the concave tentorial surfaces of the hemi- 

 spheres 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 small lobule lateral to the origins of the facial and acoustic nerves 

 is the flocculus. Lateral to this, and separated from it by a deep fissure, is the 

 paraflocculus; this is divided into dorsal and ventral parts by a sagittal fissure. 

 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 medial geniculate body is large 

 and distinct. In the deep interpeduncular fossa are two small bands which indi- 

 cate the course of the fasciculi retroflexi, tracts which connect the habenular and 

 interpeduncular ganglia. The mammillary body is double. The tuber cinereum 

 is relatively large. The hjrpophysis or pituitary body is circular and rather small. 



