Page:Encyclopædia Britannica, Ninth Edition, v. 24.djvu/202

182 VERTEBRATA TH Z ZJmC JIK FI) JZ2" 1W entirely disappeared (Cydostoma) (as has their anterior extension, the operculum, in many cases) or it has become, together with its skeletal elements, concentrated in two regions forming the pectoral and the pelvic paired appendages or limbs, with their respective girdles. The cerebro-spinal nerve-centre and the disposition of the nerves issuing from it present a remarkable complexity, and at the saine time uniformity, of structure in all Craniata (see fig. 5). The fore most of the three prim ary cerebral vesicles gives rise to paired an terior outgrowths, the prosencephala, to a median dorsal out growth, the stalk of the pineal eye (rudimentary in all exist ing Craniata), and to a me dian ventral outgrowth, which is met by an inva- gination the epidermis of the oral cavity form ing the pitu itary body, further, to a pair of lateral outgrowths, which be come the right and left optic nerves and retinae respectively. of FIG. 5. Diagrams of Craniate brain. A. Embryonic condition of neural tube. G, Cerebral portion; R, spinal cord; I, II, III, three primary cerebral vesicles. B. Longitudinal section of adult brain, applicable to any and every Craniate Vertebrate. Be, Floor of skull ; Ch, notochord ; SD, roof of skull ; NHl, nasal cavity ; VH, fore-brain, prosencephalon, or cerebrum ; Olf, olfactory lobe ; Cs, corpus striatum ; ZH, thalamencephalon, corresponding to primary anterior vesicle, from which the prosencephalon has grown out as well as Z, the epiphysis or pineal body ; I, infundibulum, with attached hypophysis or pituitary body H ; Opt, optic nerve ; Tho, thalamus opticus ; HC, posterior commissure ; MH, mesen- cephalon (corpora qiuidrigemina or optic lobes or mid-brain) ; HH, cerebellum or metencephalon ; NH, medulla oblongata or epencephalon (the reference line touches the membranous roof of the so-called "fourth ventricle" of the brain); Co, canal of the spinal cord or myelon. (From Wiedersheim.) The modifications of the hindmost of the three prim ary vesicles are also extremely definite and persistent throughout the group : its anterior dorsal surface enlarges and becomes the cerebellum (the metencephalon), whilst the cavity of its hinder part (the medulla oblongata or the epencephalon) becomes compara tively wide, and is covered dorsally by a thin membrane only, in which nervous tissue does not take a part. The intermediate primary cerebral vesicle (the mesencephalon) does not give rise to outgrowths. In all Craniata nerves are given off from the cerebro-spinal cord or tube with great regularity, one right and left in each success ive myomere or segment of the body-wall. Each nerve has two roots, a dorsal (sensory) and a ventral (motor). A commissure between each successive pair of deep or intestinal branches of the spinal nerves forms the so-called sympathetic nerve-cords, one on each side of the vertebral column. Nerves similar to the spinal nerves, but not identical with them, are given off from the brain, and perforate the cranial box right and left. In all Craniata there are ten pairs of nerves which originate thus, and in the higher forms two more pairs (elsewhere spinal) are included amongst those which thus perforate the cranium (spinal accessory and hypoglossal). The order and character of the cranial nerves are the same in all Craniata. The first (olfactory) and the second (optic) are unlike spinal nerves in both distribution and origin. As we pass back wards along the series, the cranial nerves are found to resemble more and more the ordinary spinal nerves. Hence it has been inferred that the cranial region consisted at one time of a number of distinct myomeres (as many as nine), which have become fused and modi fied to form the typical craniate "head." The oculo-motor or 3d nerve indicates the first of these segments, the trochlear or 4th nerve the second, the abducens or 6th the third, the facial and auditory (7th and 8th) the fourth, the glosso-pharyngeal or 9th the fifth, and the vagus or 10th, with certain of its branches supposed to have been originally distinct nerves, the sixth, seventh, eighth, and ninth. It is probable that in Craniata the metamerism of the gill-slits does not correspond to the metamerism of the body-wall. The mouth and each successive gill-slit are related to a bifurcate branch of a cranial nerve (furcal nerve) in lower Craniata (see fig. 3) ; these furcal nerves do not correspond, so far as we can at present judge, with cranial myomeres. The lateral pair of eyes (as opposed to the rudimentary pineal or parietal eye) present a striking uniformity of origin and structure throughout Craniata. Not only are they uniformly developed from three elements, viz., the retinal cup which grows out from the anterior of the primary cerebral vesicles, the epidermal lens which grows inwards from the surface of the skin, and the connective tissue between these two, but we find that the muscles attached to the eyeball are identical throughout the series : that is, the superior, inferior, and internal rectus, and the inferior oblique muscles repre sent the first cranial myomere, the superior oblique represent the second, and the external rectus represent the third cranial myomere. The olfactory sacs are paired in all except Cydostoma, in which they are represented by a single sac which may or may not be archaic in its azygos character. The auditory sacs are paired organs which develop as invaginations from the surface, the orifice of invagination closing up, at the hinder part of the cranial region. They present a gradually increasing complexity of form as we pass from aquatic to terrestrial forms, but are identical in essential struc ture throughout. All Craniata, except some Fishes, possess a muscular process on the floor of the oral cavity which may carry teeth, or act as a lick ing organ, or assist in suction. This is the tongue. All Craniata, with degradational exceptions, possess an outgrowth, single or paired, of the post-pharyngeal region of the alimentary canal, which is filled with gas. In many Fishes this becomes shut off from the gut ; in others it remains in communication with the gut by an open duct. In Fishes it functions as a hydrostatic appar atus. In terrestrial Craniata it is subservient to the gas-exchange of the blood and becomes the lungs. All Craniata have a large and compact liver ; and a pancreas is also uniformly present, except in Cydostoma, some bony Fishes, and the lower Amphibia. All Craniata have a thick-walled muscular heart, which appears first as an "atrium," receiving the great veins, attached to a "ventricle" by which the blood received from the atrium is pro pelled through a number of arteries, right and left, corresponding in number to the pharyngeal gill-slits between which they pass. A proeatrial chamber (the sinus) and an extra-ventricular chamber (the couus) are added to the primitive chambers ; but the most important modifications arise in consequence of the development of pulmonary respiration and the gradual separation of the cavities of the heart by median septa into a double series, a right and a left. The plan of the great arteries in all Craniata is in origin the same, and is determined by the primitive existence of a branchial circula tion in the gill-slits, which is obliterated in higher forms. Simi larly the plan of the great veins is identical, the primitive posterior vertebral veins of lower Craniata, though persistent in higher members of the group, having their function gradually usurped by the excessive development of the renal vein, and of renal-portal and ultimately of iliac veins. All Craniata have a lymphatic system or series of channels by which the exudation from the capillary blood-vessels is returned to the vascular system. It includes in its space-system the coelom and a variety of irregular and canalicular spaces in the connective tissues. Masses of spongy tissue (adenoid tissue, lymphatic glands) exist, through which the lymph filters, and there acquires corpus cular elements as well as chemical elaboration. At various points in various Craniata pulsating or simple communications are estab lished between the lymphatic system and the veins. A special and characteristic communication is established in the spleen, an organ which is found in all Craniata, either as a single mass or as scattered masses of spongy tissue in which blood-vessels arid lymph atics unite. The renal organs of Craniata are primitively a series of nephridia corresponding in number to the myotomes of the mid-region of the body in which they exist. They are connected in the simplest Craniata by a right and a left archinephric duct, which appear to be in origin lateral grooves of the epidermal surface. This primi tive renal system has been modified in some lower forms (Cydostoma and Teleostean Fishes) by the atrophy of its anterior portion. But in all other Craniata it acquires relations to the gonads or ovary and testes, so that an anterior portion of the archinephros and a corresponding longitudinal tract of the duct become separated to serve as oviduct, a middle portion to serve as sperm-duct, while a posterior portion retains exclusively or shares with the middle portion the function of urinary excretion. The male and female gonads are, with the rarest exceptions, developed in distinct indi viduals, though the rudiments of the suppressed gonad may in some cases (Amphibia) be traceable in either sex. The group of Craniate Fertebrata thus anatomically described, Sub- whilst retaining the essential unity indicated, presents an immense divisions variety of modifications. The chief modifications are distinctly of Crani- traceable to and accounted for by mechanical and physiological ate. adaptation to a terrestrial and air-breathing life, as opposed to the earlier aquatic and branchial condition. The existing forms of Craniata have been arrested at several points, in the progress towards the most extreme adaptation to terrestrial conditions, which is presented by those forms that can not only breath air and live on dry ground but fly habitually in the air. The organs most obviously affected by this progressive adaptation are the skin, the skeleton, especially of the limbs, the pharyngeal gills, and the air-bladder. This fact will appear most clearly in the subjoined classification of Craniata ; for space does not permit us to pursue further the

history of these modifications.