692 TEE CENTRAL AXIS OF THE NEBVOUS SYSTEM. 



surface. These folds, wWcli are very deep, are extremely irregular ; on the 

 surface of the hemispheres their disposition somewhat resembles the 

 convolutions of the intestines, a circiunstance to which they owe their 

 designation. In Solipeds their number is considerable, and not inferior in 

 this respect to those on the human brain. _ 



Notwithstanding their great irregularity, the cerebral convolutions offer 

 a somewhat constant arrangement ; so that it is possible to describe them 

 one by one. This has been done in human anatomy ; but it would be need- 

 less to repeat the task in the case of the domesticated animals. 



2. Mastoid or Sphenoid Lobule (Fig. 285, l). — This is a large pyriform 

 eminence, corresponding to what has been described in Man as the inferior 

 (or middle) lobe of the hemisphere, and occupies the posterior part of the 

 inferior face of the hemisphere. This eminence is curved upon itself, and 

 shows its convexity outwards. Its internal border, which corresponds to 

 the cerebral peduncle, concurs in the formation of the great transverse 

 fissure. Its large extremity is turned forward, and margined by the fissure 

 of Sylvius. The posterior extremity insensibly disappears on the inner 

 side of the posterior lobe of the hemisphere. 



This eminence ought to be considered as a large projecting convolution. 

 It is excavated internally by a csecal cavity, which constitutes the bottom 

 of the posterior or reflected portion of the lateral ventricles. 



3. Fissure of Sylvius. — Thus is designated a transverse depression 

 situated in front of the optic nerve and mastoid lobule, in which is lodged 

 the middle cerebral artery. 



4. Olfactom or Ethmoid Lobule (Figs. 321, 6 ; 322, 1 ; 327, 26 ; 

 329, 16). — -The appendage to which this name is given is detached from 

 the inferior face of the hemisphere, where it arises by two white-coloured 

 roots ; the external of these is continuous with a long convolution that 

 borders the outside of the mastoid lobule, while the internal, the shortest, 

 originates on the inner face of the hemisphere, in advance of the optic 

 chiasma. Between these roots appears a prominent surface of a triangular 

 form, constituting the extra-ventricular nucleus of the corpus striatum {substantia 

 perforata). The. appendage thus formed passes forward, terminating 

 in an oval enlargement (hulbus olfactorius) extending much beyond the 

 anterior extremity of the brain, to be lodged in the ethmoid fossa. 



The olfactory lobe possesses an internal cavity, a diverticulum of the 

 lateral ventricle (Fig. 322, 2). Both lobes being regarded as the first pair 

 of cranial nerves, we will return to their description when studying the 

 encephalic nerves. 



INTERNAL OONFOKMATION OF THE BRAIN. 



In separatingt he cerebral hemispheres by their upper face, we discover 

 the great commissure known as the corpus callosum : the first object that 

 presents itself for study in the internal conformation of the brain. 



If we afterwards remove, by a horizontal section, and with a sharp 

 instrument, all that portion of the hemispheres which covers this commissure, 

 and also if the latter be excised to a certain extent to the right and left of the 

 median line, we will penetrate two symmetrically disposed cavities in 

 the centre of each hemisphere. These cavities are the lateral or cerebral 

 ventricles. 



They are separated on the middle plane by a thin partition — the septum 

 hcidum, which is attached to the corpus callosum by its upper border, and 

 fixed by its inferior border into the fornix, a kind of median arch beneath 



