The general pattern of the nervous system and coordination in Rana temporaria is the same as that of the dogfish in all essential respects, the main differences being those connected with the structural changes undergoes by the animal in correlation with terrestrial life.
The Brain And Spinal Cord
In the telencephalon, the small olfactory lobes project forward and are secondary united in the mid-line. The cerebral hemispheres are clearly defined, the right and left being partially separated by a deep dorsal furrow. Internally the corpora striata are well developed.
The thalamencephalon is narrow, its roof consisting of the anterior choroid plexus, behind which is the pineal stalk. In the tadpole, the pineal body is attached to its stalk, but in the frog, fronto-parietal ossification has separated them and the pineal body now lies beneath the skin between the eyes. Sometimes it is visible as a pigmented spot.
In some of the extinct amphibians there was a large fontanelle in the cranium and it is probable that they possessed a pineal eye connected to the brain by the pineal stalk which acted as an optic nerve. Ventrally, there are the usual piturtary body and optic chiasma. The pituitary consists of the infundibulum and the non-nervous hypophysis; there are no lobi inferiors or sacci vasculosi. Internally, the thalami are relatively larger than those of fishes but the hypothalami are not so well developed.
In the mesencephalon, the optic lobes are large and conspicuous ovoid protuberances, and ventrally the pituitary overlies the crura cerebri.
The outstanding feature of the metencephalon is the small size of the cerebellum, a narrow transverse strip in front of the large triangular posterior choroid plexus. Ventrally the medulla is thickened and begin to taper posteriorly.
The mylencephalon consists of the posterior choroid plexus dorsally and the medulla laterally and ventrally. The latter tapers into the spinal cord and in this region the two are hardly distinguishable.
The spinal cord is not of uniform cross-section, being somewhat swollen where the nerves to the limbs are situated. It begins to tapper at the level of the sixth vertebra and ends as the filum terminale in the urostyle. In transverse section, it has the same characteristic as are found in the dogfish but it is somewhat more flattened dorso-ventrally.
Cavities Of The Central Nervous System
There are two narrow rhinocoels in the olfactory lobes which communicate with the two large lateral ventricles of the cerebral hemisphere. These lead, by the foramen of monro, into the third ventricle which is somewhat reduced by the large lateral thalami. Optocoels open into the constricted aqueduct of sylvius which leads into the large fourth ventricle beneath the posterior choroid plexus. The narrow central canal of the spinal cord is continuous with the cavity of the fourth ventricle.
Commissures and Correlation Centres
Further study of the nervous system and coordination in rana temporaria reveal that the lateral commissures, the anterior in the lamina terminalis is well developed, consisting of dorsal and ventrally tracts of nerve fibres.
The dorsal tract connects groups of cells in the cerebral hemisphere, while the ventral tract connects cells in the lateral walls of the hemispheres concerned with reception of sensory impulse from the olfactory lobes. The dorsal commissure in front of the pineal stalk is less well developed; it connects two small masses of cells again concerned with the olfactory sense. Connecting the antero-ventrally region of the two optic lobes is the posterior commissure which is more prominent than in fishes. Than crura cerebri are well developed, containing large tracts of fibres connecting the sensory centres of the hind-brain with those of the fore-brain.
There is less emphasis on the cerebellum as a correlation centre compared with its great importance is fishes. Stimuli relating to balance and orientation tend to be diverted into the posterior region of the optic lobes and there is thus a very small cerebellum and a reduced length in the medulla. The thalamus is well developed, receiving connexions from all the sensory centres. There is some development of the pallium, a dorsal region of superficial grey matter in the cerebrum. It is connected to all parts of the brain but has not yet reached the overriding importance it attains in the mammals. Removal of the cerebral hemisphere seems to have little effect on behavior except that concerning olfactory stimuli.
Relationship of the Brain to Mode of Life
It must be emphasized that when we attempt to correlate the anatomy of the brain with the animal’s mode of life(Rana temporaria), we can do so only by comparison with brain structure in other animals. When we say that the olfactory lobes are small in the frog, indicating less reliance on the sense of smell, we are comparing their relative size with the same structure in the fishes.
The cerebral hemispheres show great development compared with those of the dogfish. This indicates the trend in terrestrial animals of greater centralization of control in this region and hence greater possibilities of more complex behaviour. Relative enlargement of the optic lobes indicates the greater importance of the sense of sight. The great reduction of the cerebellum may be correlated with the decrease of muscular activity. The frog is sluggish by comparison with the fish and it is more or less stationary for the greater part of its life. Its broad body, touching the ground, ensures stable equilibrium without much necessity for fine and continuous adjustment of the muscle concerned with balance. During jumping, the frog has little judgement of distance or direction; it will jump anywhere and it does not choose a spot for landing, it merely falls to the ground. The smaller length and thickness of the medulla are correlated with the loss of the lateral line system, which is so important in the fishes.
Cranial and Spinal Nerves
As in the dogfish, there are ten pairs of cranial nerves, though in some extinct amphibian there were probably twelve pairs. Thus the condition in the frog is secondary, not primitive. The main changes in distribution of the cranial nerves are related to the loss of the lateral line system and of the gill clefts. The nerves which supply these structures in the dogfish are VII, IX and X; these have undergone most change in the frog.
Each olfactory nerve I, leads the fibres from a nasal organ into an olfactory lobe; it is a single small nerve II, III,IV, V, VI and VIII have the same distribution as in the dogfish. In connexion with the distribution of VII, it must be remember that the Eustachian tube corresponds to the spiracle of the fish. There is a palatine branch to the roof of the mouth, a small pre-tympanic in front of the Eustachian tube and a large hyomandibular behind it. This last branch supplies the muscles attached to the hyoid body and the muscles of the lower jaw. Cranial nerve IX now lives up to its name of glosso-pharyngeal; it passes forward beneath the pharynx to the tongue. Only the visceral branch of the vagus remains, the branchial and lateral line branches having disappeared; it has laryngeal, cardiac, pulmonary and gastric derivatives.
There are ten pairs of spinal nerves, the first nine emerging from the intervertebral foramina and the tenth pair from small foramina in the urostyle. The dorsal and ventral roots unite outside the vertebral column the junctions being obscured by white calcareous masses known as the gland of schwannerdam. Each mixed spinal nerve give off a short dorsal branch to the skin and muscles of the back, a large ventral branch to skin and muscles of the abdomen, and where applicable, to the limbs, and a short ramus communicans to the autonomic chain.