Dissected so as to show its craniospinal nervous axis in its entire length as well as portions of most of the organs of vegetative life.
A RED injection has been thrown into the veins; and the left halves of the walls of the craniospinal, thoracic, abdominal, and pelvic cavities, as well as the greater part of the integument in the facial region and the greater part of the left lung, have been removed so as to show in situ the organs previously concealed by these structures.
Of the encephalic nerve-centres we see most anteriorly the olfactory lobes: next to them the cerebral, separated from each other by the longitudinal fissure in which is lodged the longitudinal sinus: next the cerebellum bounded off anteriorly from the posterior border of the cerebral lobes by the diverging lateral sinuses, into which the longitudinal sinus divides. The presence of the lateral sinuses prevents us from seeing the corpora quadrigemina which would otherwise be visible in the middle line, owing to the divergence there from each other of the cerebral lobes. The medulla oblongata, which is, like the cerebellum, of considerable width, comes into view between the two occipital condyles, from which point down to the second dorsal vertebra, recognizable by its long spine carrying an ossicle articulated to its apex, the medulla spinalis is of much greater thickness than it attains posteriorly. It is seen in the lumbar region to break up into the cauda equina.
In the dorsal region, a black bristle has been passed under the aorta where it underlies the bodies of the vertebrae, and this position relatively to the craniospinal canal superiorly, as also to the digestive tract next in-feriorly, and the heart most inferiorly, is held by the aorta in all vertebrata. The singleness of the aortic trunk in the adult state is characteristic of all warm-blooded animals; but mammals, as is seen here, differ from birds in having the single trunk arching from the heart over the left and not over the right lung's root. Behind and to the right of this black bristle from before backwards are to be seen, firstly, the fourth lobe of the right lung in its pleural cavity resting on the diaphragm below, and in relation above with the heart, and on the left with the phrenic nerve; secondly, the oesophagus, a lowly vascular tube the small calibre of which is correlated with the working of the dental apparatus in these creatures; thirdly, the third lobe of the right lung placed far back and to the right, and, like the lungs of all mammals, freely suspended in its pleural cavity and bearing no impressions on its exterior from the different bony constituents of the thoracic cavity; fourthly, the vena azygos of the left side between the aorta and the vertebral column, passing up to arch over the root of the left lung, and join the vena cava descendens of that side; and fifthly, the spinal cord.
The complete diaphragm, forming a dome-shaped floor, with the heart and lungs in relation with its convex, and the liver, stomach, spleen, and kidney in relation with its concave surface, is eminently characteristic of Mammalia, that of the Crocodilina alone approaching this grade of development. The upper part of the pericardial sac has been removed, and the two ventricles (less distinctly separated from each other than in many mammals) and the left auricle are brought into view. The anterior surface of the heart is more equally shared in by the two ventricles than is the case in many mammals, in which the right ventricle forms nearly the entire anterior aspect of the organ. The left vena cava descendens, a trunk which is found in most Rodents, except the Guinea Pig and Agouti, is seen to pass in front of the root of the left lung in company with the phrenic nerve round to the back of the heart to end in the right auricle. The vena azygos of the left side is seen to join it just above the root of the left lung, and at a point some way above this, the vein from the fore-leg, which is in relation with the nerves going to that limb, is seen passing up to join another vein, which, from its being placed superficially to the sternomastoid muscle, we know to be the homologue of the external jugular of anthropotomy.
The external jugular is the main trunk by which the blood from the interior of the skull returns to the heart in the Rodents and many of the lower Mammalia, and by its confluence with the vein from the anterior limb the vena cava descendens is constituted. Internally to the external jugular, just above its confluence with the subclavian vein, is seen a part of the hibernating gland; externally to it lies the submaxillary; above this again we see the parotid with its duct; and above the parotid, the facial portion of the lacrymal gland sending up a duct, under which a piece of blue paper is placed, to enter the orbit and join there with the duct of a second portion of the lacrymal gland, which is placed within the orbit, and anteriorly to the duct of the extra-orbitally-placed portion. Within the orbit we see the Harderian gland in relation with the third eyelidl.
1 For a fuller description of these glands, see Description of Plate I, which represents a dissection somewhat different from that which we have of these organs in this preparation. The lacrymal gland is somewhat similarly bilobed in the human subject, consisting of a palpebral and an orbital part. See Hirschfeld et Leveille, Neurologie, 1853, PI. 76, fig. 4. In man however there are between twelve and fourteen minute lacrymal ducts instead of a single one as here. But the macroscopic Harderian gland and duct of the Rodentia and mammals lower than Primates except the Chiroptera do not similarly represent the minute Meibomian glands with separate ducts on the free edge of the eyelids; for both sets of glands coexist in Rodentia.
In the middle line of the body inferiorly to the heart we see the cut surfaces of the six sternal bones, and in the angle intercepted between the lowermost of these and the diaphragm, we see some lobules of fatty tissue set in the process of serous membrane which connects the apex of the pericardium with the sternal bones and with the diaphragm. From these structures a vein passes back along the pericardium to end in the vena cava descendens of the left side.