778 THE CENTRAL AXIS OF TEE NERVOUS SYSTEM. 



small protuberential masses, disseminated between tlie transverse fibres of this 

 region ; and the locus niger — a grey mass that separates the two stages of the 

 crura cerebri. Finally, there is a small mass of this grey substance which con- 

 stitutes each of the corpora quadrigemini, and which is covered by a thin pellicle 

 of white matter, scarcely visible in the anterior eminences. 



The fhalami optici is a similar mass, though more volummous, darker coloured, 

 and without a layer of white substance on its superficial face. 



Lastly, nerve-cells exist between the various layers of transverse fibres of the 

 pons Varolii, and between the tubes which constitute the valve of Vieussens. 



Differential Characters in the Isthmus op the other Animals. 



Apart from its volume, the isthmus does not present any sensible differences in Rumi- 

 nants and the Pig. In the Ox, it is remarked tliat : 1. The inferior pyramids of the medulla 

 oblongata are more prominent, and the transverse cords parallel to the pons Varolii more 

 voluminous than in Solipeds. 2. The crura cerebri are short. 3. The optic nerves are larger 

 than in Solipeds. 4. Tliere is a large developed pituitary gland, excavated by a wide cavity, 

 and flattened above and below. 5. Lastly, the testes are more conical, and less distinct from 

 the nates than in the animals already studied. 



In the Carnivora, the fourth ventricle is very wide and deep, and bordered by salient 

 and detached corpora restiformia. Its floor is marked by some white transverse striae, more 

 apparent than in the Horse. The pons Varolii is large ; the columns of the medulla oblongata, 

 parallel to its posterior border, are as developed as in the Horse, without taking into con- 

 sideration the difl"erence8 in size of the two species. The pyramids are voluminous, and the 

 olivary bodies well defined. The testes are larger than the nates. 



Asa general rule, the development of the pons Varolii is in relation to that of the lateral 

 lobes of the cerebellum. 



Comparison of the Isthmus of Man with that of Animals. 



In human anatomy, the medulla oblongata and isthmus are described separately. 



The first shows on its lower face a well-marked groove — a continuation of that of the spinal 

 cord. It terminates anteriorly in a deep fossa, named the foramen cascuin of Vicq-d'Azyr. 

 The pyramids are well marked. The olivary bodies are much more prominent than in animals, 

 and are also distinguished by the presence of a grey nucleus in tlieir interior. The medulla 

 oblongata of Man has not the transverse band, behind the pons Varolii, which we have found 

 in the Horse (Fig. 424). 



With regard to the isthmus proper, it contains the parts in front of the medulla oblongata 

 already studied in the domesticated animals. Tlie pons Varolii is very large ; the crura cerebri 

 are separateil from each other by a groove, at the bottom of which are several small openings. 

 The fourth ventricle is deep, is bordered by well-developed corpora restiformia, and enclosed 

 posteriorly and laterally by the valves of Tarini (velum medullare posterius). On its floor are 

 remarked transverse strise (lineas transversae) named the barbs of the calamus scriptorius, which 

 are also found in the Dog. The testes are smaller than the nates; but the diflerence in their 

 volume is less considerable than exists between Solipeds and Ruminants. Their structure is 

 identical with that already described. 



Article III. — The Cerebellum. 



The cerebellum, or posterior enlargement of the brain — is the single mass 

 supported by the isthmus, separated from the cerebrum by the transverse parti- 

 tion constituting the tentorium cerebelli, and lodged in the posterior compart- 

 ment of the cranial cavity, which almost exactly gives the measure of its volume. 



Leuret has ascertained the weight of this organ. On the average it weighs 

 2i ounces in the entire Horse and Mare, and 2| ounces in the castrated Horse. 

 This weight is to that of the brain as 1 : 7"07 in Stallions ; as 1 : 6-59 in 

 Mares ; and as 1 : 5'97 in Geldings. 



