g OM THE NERVOUS SYSTEM IV. 



Amphioxtis, on the other. The spinal chord in the Anourous species exhibits the 

 general peculiarities of the division, and in addition the enlargements correspond- 

 in" with arms and legs ; while the brain is so far reduced in the relative proportion 

 of its different parts, and so far stripped of the " accessory organs of perfection," 

 as to enable the student to obtain with ease a clear conception of the general plan, a 

 conception always so difficult to acquire when studying the brain of Mammals or of 

 Man. In these, the cerebrum and cerebellum so far transcend all the other organs 

 of the encephalon, that the parts which in a morphological point of view are of 

 equal value have been frequently overlooked, as forming either integral parts or 

 ijrimary subdivisions. In Frogs, in common with the lower Vertebrates, while no 

 one part takes an excessive development, there is at the same time no one of the 

 fundamental ones either wholly deficient, or so far reduced as to deprive the general 

 plan of any of its more important features. 



In making a dissection of the cerebro-spinal axis, when the canal in which it is 

 lodged is laid open, one of the peculiarities which first attracts the eye is the ex- 

 istence of a whitish substance deposited in a somewhat irregular manner, but prin- 

 cipally on the sides and around the veins, extending through the whole length of 

 the canal and into the cranium, where it is especially abundant in the neighborhood 

 of the optic lobes. Occasionally it will be found in such quantities as materially 

 to embarrass the anatomist in his dissections, and at other times only traces can be 

 detected. Under the microscope this substance is resolved into vast numbers of 

 minute crystalline forms, consisting of calcareous matter, and similar to those here- 

 after to be described in connection with the spinal nerves, where it is found lodged 

 in peculiar capsules, and in Avhich the quantity is more constant. 



After the chord is completely exposed, one of its most striking features, when com- 

 pared with the same part in other Reptiles, except Anourous Batrachians, is its short- 

 ness. (Plate II. Fig. 1.) The point at which the last pair of spinal nerves is given 

 off is just beyond the middle of the trunk, that is, at about the seventh or eighth 

 dorsal vertebra. Beyond this the chord is continued only in the form of a slender 

 thread, which has been compared to a ligament, though it contains the microscopic 

 elements of the chord, but at the same time giving off no nerves, and is lodged and 

 terminates in the long and slender coccyx. In other Reptiles, on the contrary, in 

 Saurians, Chelouians, Ophidians, and Urodel Batrachians, the chord is continued as 

 such to the extremity of the spinal canal, giving off its pairs of nerves nearly opposite 

 to the intervertebral foramina which correspond to them. While it has this excep- 

 tional shortness in the adult of the Anourous species, it is of great interest to notice 

 the fact, that, in their embryonic condition, tliey conform more nearly to the general 

 rule, as was long since shown by Serres,* by numerous other observers after him, 

 and as will be seen in the sequel of these pages ; nevertheless, as will be shown 

 hereafter, this caudal portion does not conform precisely to the true spinal type. 

 The subdivisions of the central axis into brain and spinal chord, and of these into 

 subordinate parts, are enumerated in the following tabular view. 



* E. R. A. Serres, Anat. Comp. du Cerveau dans les Quatres Classes dcs Anim. Verteb. Paris, 

 1829. 



