Feb. 19, 1874] 



NA TURE 



307 



that the correspondence between the brain of the frog and 

 that of man becomes so disguised through the unequal 

 grovvtli of different portions of the organ in the human 

 brain as it advances in its growth and development. The 

 same six successive portions, however, exist in each. 



1. In the frog the olfactory lobes acquire a much larger 

 relative size, and they retain permanently an internal cavity 

 which exists only transitorily in man. 



2. The cerebral lobes (or hemispheres) exceed those just 

 noticed but are insignificant indeed, when compared with 

 the corresponding human structures. They may, however, 

 be more insignificant than in the frog, as, for example, in 

 the lamprey, where they are actually smaller than the 

 olfactory lobes. In that the cerebral lobes of the frog each 

 contain a cavity (the lateral ventricles) they have a charac- 

 ter which is constant in all animals above fishes, they open 

 by a common aperture (foramen of Monro) into the cavity 

 of the next brain segment behind. 



3. This third segment retains a great relative magnitude 

 compared with that of man. 



4. The fourth segment, however, consisting of the optic 

 lobes, attains a still further relative development, though 

 consisting only of two bodies instead of four, but these 

 contain a cavity not found in the corpora quadrigemina of 

 the human brain. 



5. The fifth segment, the cerebellum, is very small, and 



Fig. 72.— Brain of Bull Frng in various views i, Dorsal view. 2. Literal 

 view : 3, Transver.e horizontal section showing the cavities of the olfac- 

 tory cerebral and optic lobes. 4, Longitudinal section a little to the left 

 of the median line. 5, Longitudinal section in median line. The 

 corpus striatum, r, is here exposed to view and also a body, g, within 

 the optic lobes. 5, Longitudinal section in med .an line. In all five 

 figures: — I, Olfactory nerve; 2. optic nerve ; 4. auditory nerve: a, 

 olfactory lobe ; /', cerebral lobe : f, corpus striatum ; d^ op'ic thala- 

 mus ; c, pineal gland ;/, pituitary body : g, optic lobes ; h, cerebellum. 



smaller than the same part in animals both higher and 

 lower in the scale ; indeed, in the frog class, this organ may 

 be said to be at its minimum. When cut it exhibits no 

 trace of an nrhor I'ita-- 



This fact has a special interest as bearing on alleged 

 functions of this portion of the brain. 



It has been asserted by some that the cerebellum minis- 

 ters to the sexual functiors, by ethers that this part co- 

 ordinates and directs locomotive movements, and, quite 

 lately, that it is related to movements of the eyes. 



The first two of these hypotheses seem to be completely 

 overthrown by our frog. In the first matter there is any- 

 thing but a deficiency of energy and activity, and as to 

 the second, many reptiles are less active and continuous 

 than the frog in their locomotive efforts. As to the third 

 hypothesis, it should be remembered that the eyes of the 

 Frog are large and very moveable, as also that they re- 

 quire a power of ready adjustment to enable the animal to 

 secure its insect prey. 



6. The sixth and last segment of the brain, the medulla 

 oblongata, is also relatively large, and is exposed to view 

 through the rudimentary development of the cerebellum 

 which, as has been said, overlaps it in man . ^-^-^ 



It has been already said, that in man and the higher ani- 

 mals there are nerves supplying the orbital muscles and 

 different parts of the face. 



The eyeball in man is moved by six little muscles, four 

 straight, (the recti) and two oblique, one being the upper, 

 the other lower, oblique. 



Now a nerve called the third, because it follows the first 

 two (olfactory and optic) goes from the brain to all the 

 orbital muscles except the upper oblique and the outer 

 rectus. 



Another nerve, ^t fourth, proceeds to the upper oblique 

 muscle only. 



The fiftJi iti-rvc is a very large one, and supplies the nose, 

 tear-gland, eyelids, upper and lower jaws, tongue and teeth. 

 ^ The sixtli nerve is a very small one indeed, being exclu- 

 sively applied to the outer rectus muscle of the orbit. 



The seventh nerve is, in part, the auditory nerve in part 

 it sends fibres to the face. 



The eighth nerve is a very complex structure, and con- 

 sists of, at least, three nerves united together, all arising 

 from the medulla oblongata. It sends branches to the 

 parts about the throat, as well as to the organ of voice, 

 to the lungs, the stomach and the heart. 



The nerves of the frog exhibit certain intermediate con- 

 ditions lilce those we have seen to exist in various other 

 parts of its anatomy. 



In the higher vertebrate animals, as in Man, the 



J. th" Eyeballs, M.>wed fro-n obn\e .-utcI from the 

 :sutjerior rectus ,In/ li , the infei lor rectus ; E.R.^ 

 the external ret-tus , In K , t'le internal rectus , S Od , the superior 

 oblique , /n/Ot> , the inferior oblique . Cti , the chiasma of the optic 

 nerves (// } . /// , the third nerve, which supplies all the muscles ex- 

 cept the superior oblique and the external rectus. 



muscles which move the eye-ball are supplied by three 

 distinct nerves termed respectively the 3rd, 4th, and 6th. 

 The 5th nerve being a very large and complex one, send- 

 ing branches to various parts of the head and its organs. 



Now in the frog there is no distinct 6th nerve, it being 

 replaced by an extra branch of the 5 th nerve. This 

 modification, however, is but one step towards a condition 

 which obtains in the Mud-fish [Lepidosiren), when all 

 these three nerves are quite blended with one division 

 (the Ophthalmic) of the fifth nerve. 



Again in the higher Vertebrates, as in Man, the 8th 

 nerve is a very large and complex one, and distributed as 

 in him. It is also so distributed in the adult frog. 



In the tadpole, however, this nerve shows a very diffe- 

 rent arrangement. Alter issuing from the skull this nerve 

 sends a branch down the outer side of each branchial 

 arch and then gives off a very long one, which extends 

 laterally, i e. along the side of the body and tail. 



Nothing like this exists in any Beast, Bird or Reptile, 

 but when we come to the class of Fishes we encounter a 

 precisely similar state of things. Here we find the eighth 

 nerve sending a branch to each branchial arch, and 

 giving off a great nerve proceeding along the side of the 

 body and tail, and on that account named the nervus 

 lateralis. 



^,., St. George Mivart 



{To be continued.) 



