460 REPTILES LA. NATO the sympathetic system. Reptiles also generally possess three distinct organs of sense (1) ears, (2) eyes, and (3) nasal organs though one or more of them may be excep- tionally rudimentary and defective. Axial The, Neural Axis. The spinal marrow extends through portion, nearly the whole length of the neural canal of the skeleton in the form of a long nervous cylinder with a small central cavity, with a deep furrow along the middle of its dorsal and a shallow one on the middle of its ventral aspect. It becomes more or less augmented in volume about the region of the shoulders and loins, where the nerves of the limbs are given off from it. Even where most enlarged, however, the tracts on either side of the dorsal furrow do not diverge and leave a space or sinus as they do in the lumbar enlargement of Birds. Where the spinal marrow comes to join the brain it enlarges considerably. At the medulla oblongata it is bent down at a marked angle with the more posterior part of the neural axis. The two halves of its dorsal portion also diverge so as to expose the dorsal surface of the more ventral portion in a space which is known as the fourth ventricle or ventricle of the medulla. This medulla advances forward, expanding and becoming locally differentiated to form the brain, its ventricle being continued on into a more anterior and more completely enclosed cavity known as the third ven- tricle. The nervous roof of the hinder part of this latter ventricle is produced into two smooth prominences placed side by side and called the optic globes or corpora bigemina (which may themselves contain a cavity), immediately behind which is an azygous nervous structure, the cerebel- lum, which more or less roofs over the hindmost or fourth ventricle. In front of the optic lobes are the larger rounded smooth neural masses the cerebral hemispheres, each of which contains a cavity the lateral or first and second ventricles, which both communicate with the anterior end of the third ventricle through an aperture termed the foramen of Munro. Between the hinder ends of these hemispheres an azygous structure projects upwards the pineal gland while from beneath the floor of the third ventricle another azygous structure projects downwards the pituitary body. In front of the hemispheres are the other rounded and smaller neural masses, the olfactory lobes, which are generally elongated and contain cavities that are continuations forwards of the lateral ventricles. Three transverse bands of nervous tissue connect the struc- tures which bound the third ventricle laterally ; the first of these, placed just behind the lamina terminalis or the front boun- dary of that ven- tricle, is called the anterior com- missure, and the nervous masses it joins are the COr FIG. 23. Rraln of LaceHa agHis (after Leydig). 1, r>r>ri ctriata An I'orsal aspect ; 2, vertical lonKitudin;il section. r&, " cerebellum; eh, cerebral hemisphere; m, medulla Other more pOS- oblongatn; olf, olfactory lobes; on, optic nerve; opt, i . , , j optic lobes; p. pineal gland ;py, base of pituitary body. tenorly situated band is called the soft commissure ; and the third, a little farther back still, is the posterior commissure. The masses joined by those last two commissures are called the optic thalami. The brain is invested with membranes in the same general way as it is in Mammals. In the Lasertilia tlic cerebellum is a thin very small body projecting rather upwards, and may he transversely segmented, as in Platydactyl'us. In the Chameleons the optic lobes and hemispheres are of nearly equal size, and the olfactory lobes nre neither separated off nor hollow. In the Ophidia the cerebellum nearly covers the fourth ventricle, the hemispheres are of consider- able size, and the olfactory lobes may be immediately annexed to them. In the Chelonia the cerebellum is rather large, but it is only in the Crocodilia that it is marked by transverse grooves, as is the central part of the cerebellum or dermis in Mammals ; the hemispheres also are of considerable volume. In some extinct Rcptilia, e.g., in the Dinosaurian Stcgosaurus, the brain was exceedingly small, probably weighing but the hundredth part of that of the Alligator compared with the weight of the bodies of the two animals. Moreover, the cerebral hemi- spheres but little exceeded the optic lobes in size, while the transverse diameter of these lobes only slightly exceeded that of the medulla. The cerebellum also was very small. In the Ornilhosaiirin, how- ever, the optic lobes were unlike in their situation those of any existing Reptiles, they being depressed and placed laterally instead of on the dorsum of the brain, thus resembling the optic lobes of Birds. The Peripheral System. As in the higher Mammals, so Pa also in Reptiles, the neural axis gives off a series of special r hi nerves, which in a general way correspond in number with ^ T * the vertebrae they pass out between. Each also arises by a superior and inferior root, whereof the former is furnished with a ganglion. After the junction of these two roots the whole nerve thus formed divides into ascending and descending branches the latter, in the trunk, running between the internal oblique and transverse muscles. The cranial nerve which comes forth from the brain nearest the spinal marrow is the hypoglossal. It passes out through a condy- loid foramen in the occipital bone, and goes to the tongue, hyoid, and larynx. A more anteriorly placed nerve, called the spinal accessory, exists except in Ophidia. It arises between the superior and inferior roots of the more anterior spinal nerves, and then enters the cranium, which it leaves again in company with that next to be noted. It supplies certain dorsal muscles of the neck or anterior part of the trunk. The next or pneumogastric nerve passes out into the spinal accessory. It is distributed to the lungs, stomach, and heart. To these, next in advance, succeeds theglosso- pharyngeal for the tongue and pharynx. Very distinct from the foregoing is the facial nerve, which passes out at a distinct foramen and turns rather backwards to supply the sides of the head. Next is to be distinguished the acoustic nerve, which goes to the internal ear, and in front of that again is the trigeminal, which is wont to make its exit at two distinct places, and diverges into three branches, which go respectively to the orbit, the upper jaw, and the lower jaw. Then come three small nerves destined to supply the muscles which move the eyeball ; and foremost but one is the optic nerve; this crossing its fellow of the opposite side beneath the brain, the fibres of the two nerves blend at their point of intersection and so form what is called the chiasma. Most anterior of all are the olfactory nerves, which proceed from the olfactory lobes to the nasal organs themselves. In the Chelonia the spinal accessory may take origin as far back as the root of the fourth cervical nerve. The pneumogastvic and glosso-pharyngeal pass out from the skull separately as in Saurians, and not through one foramen as in Ophidians and Crocodilians. Those spinal nerves which continue to supply the limbs form certain more or less complex unions with each other termed plexuses, and from each plexus the nerves of the fore or hind limbs, as the case may be, proceed. The plexus which supplies the nerves of the fore limb is called the brachial plexus. It is formed by the blending of about the sixth, seventh, eighth, and ninth spinal nerves in the Turtle and Crocodile, and the nerves it gives forth are tho axillary, ulnar, radial, musculo-spiral. and median as in Mammals. Certain of the more posterior nerves unite to form a crural plexus and others just behind the former blend to form a sacral plexus, and from these the nerves of the leg and pelvic region proceed. Amongst these nerves are the crural, obturator, and especially the great sciatic nerve, which passes to the back of the thigh and divides into the libial nerves. As to the particular spinal nerves which go to form these plexuses respectively, and as to the mode of their interlacement and mode of giving origin to the limb nerves, there is not only diversity between different genera of the same order and species of the same genus, but also between different individuals of the same genus, and even between the two sides of the same individual Keptile. The Sympathetic Nervous System nerves which copiously supply the viscera is least marked in the Ophidia, in which p> " this system is not distinct, in the greater part of the body, ' " from the spinal nerves. Essentially the system consists of two antero-posteriorly extending nervous cords, which run on eithiT side of the axial skeleton in its ventral aspect. Each of these two cords is joined by fibres from the spinal and cranial nerves adjacent to it except the nerves of the nose, eye, and ear. At the points of
