HEAD AND NECK OF THE HORSE 171 



the cervical region and the first part of the thoracic region, the dorsal 

 funiculus is subdivided into a fasciculus gracilis and a fasciculus 

 cuneatus by the dorsal intermediate groove. The fasciculus gracilis, 

 always narrow on the surface of the cord, disappears entirely about the 

 middle of the second cervical segment, and often does not appear agam. 

 Occasionally, however, a glimpse of it may be obtained in the medulla 

 oblongata of the brain, at the posterior angle of the fourth ventricle. 

 At the point of disappearance of the fasciculus gracilis, an additional 

 ridge-like area (tractus spinalis nervi trigemini) appears on the 

 surface of the cord, narrow at first, but gradually widening. This 

 is formed by the substantia gelatinosa covered by fibres of the spinal 

 root of the trigeminal cerebral nerve, and later, during the examination 

 of the brain, will be found to end at the tuberculum cinereum of the 

 medulla oblongata. 



The brain (Encephalon).— Before beginning the actual dissection 

 of the brain, it will be profitable to make a general survey of its 

 external conformation. The whole organ lias an irregularly ovoid 

 shape, in conformity with the cranial cavity in which it is lodged. 

 In weight it averages some 650 grammes. Its posterior end is formed 

 mainly by the medulla oblongata and the cerebellum, but partly also 

 by the hemispheres of the cerebrum. The anterior ends of the hemi- 

 spheres, capped by the olfactory bulbs, form the narrow extremity of 

 the ovoid. Of the various component parts of the organ, the cerebral 

 hemispheres constitute by far the greater proportion of its entire bulk, 

 and it is these that contribute the greatest transverse diameter. 



Viewed from the dorsal aspect, the cerebral hemispheres are the 

 most conspicuous objects, though part of the cerebellum and a short 

 length of the medulla oblongata may also be seen. The two cerebral 

 hemispheres are separated from each other by a longitudinal fissure 

 (fissura longitudinalis cerebri), and a transverse fissure (fissura trans- 

 versa) divides them from the cerebellum. The surface of each hemi- 

 sphere is marked by grooves and convolutions arranged in what appears 

 at first glance to be an inextricably confused pattern ; but a closer 

 examination at a later stage of the dissection will show that all the 

 grooves are not of the same depth nor of equal morphological value. 

 The surface of the cerebellum is also covered by grooves and ridges, but 

 here they run mainly in a transverse direction ; those of the cerebrum 

 being chiefly longitudinal or oblique. The cerebellum, moreover, is 

 divided by rather shallow and ill-defined antero-posterior depressions 

 into a middle vermis ^ and two lateral heinispheres. 



1 Vermis [L.], a worm ; fiom the resemblance of this part of the human cere- 

 belhim to a caterpillar. 



