Page:The American Cyclopædia (1879) Volume V.djvu/180

 176 COMPARATIVE ANATOMY as the diminution of their innocent beauty as age advances. Those animals which have the lowest heads and the longest snouts are always considered the most stupid and gluttonous, as the hog among quadrupeds ; as we descend to reptiles and fishes, it has been seen that the jaws constitute almost all the head, and these are known to be the most voracious of crea- tures, apparently living only to eat; on the contrary, a great degree of intelligence is at- tributed to the elephant from his well marked forehead, and the perpendicular-visaged owl is made the companion of the goddess of wis- dom; though in the last instance these sem- blances do not depend on any greater devel- opment of brain, but are mere bony expan- sions. Even among men, we instinctively re- gard him as stupid and sensual whose face is very prominent and whose forehead is re- ceding; the advancement of the forehead to- ward the line of 'the face is always understood by artists as representing a noble and elevated character. As we recede from man the brain cavity diminishes, the jaws and nasal fossaa are lengthened, the orbits are directed more exter- nally and are less distinct from the temporal fossaa ; the occipital foramen and its two con- dyles gradually fall behind the middle of the base of the skull, and finally occupy its poste- rior face, so that the jaws, instead of being at right angles to the spine, become parallel to the axis of the body. The eight cranial bones of man may be recognized in the mammal skull, though they are variously subdivided in the different families, and in some are united to- gether ; the two parietals are united in the car- nivora, while the frontals remain separate, and the temporal tympanum is divided from the rest of the bone by a suture ; in the elephant the frontals and parietals are early united with the other bones into a solid box. Though the skull of the highest apes resembles that of man in shape, the bones are differently connected ; the wing of the sphenoid does not reach the parietal and barely touches the frontal, and the temporal suture is serrated rather than squa- mous. From the position of the occipital fora- men it is evident that the head of mammals is not balanced on the spine, but is suspended from the neck and back by the ligamentum nnche. The bones of the face differ from the human in the greater number of pieces and in their horizontal extent. In man the upper jaw bones contain all the upper teeth ; but in the lower animals the incisors are contained in an intermediate bone, the intermaxillary, a per- sistence of a separation which may be detected in the human foetus. The palate bones are small in the carnivora, and large in the rodents; the upper jaw is elongated in all. The pecu- liarities of the bones forming the orbits will be given under the heads of the different families. No animal but man has a chin ; in all below him the anterior arch of the lower jaw is con- vex vertically and retreating at its lower mar- gin; in the whale it resembles two immense ribs, united at the points, without any ascend- ing branches, the articular surface being di- rected backward ; in the hare and rabbit the coronoid process, to which the elevating mus- cles are attached, is small, but in the squirrel and rat it is large, and the condyle or articular process is compressed laterally and largest in front ; in carnivora the condyle is transverse, and admits only of a hinge-like movement ; in the ruminants the flat articular surface allows considerable lateral motion; in cetacea and edentata there is neither ascending ramus nor coronoid process, and the angle formed by the body and ramus is gradually reduced until it becomes on a line with the axis of the jaw ; in carnivora, rodents, and ruminants, the two sides of the lower jaw are never firmly united at the symphysis. Many mammals have the head surmounted with horns ; that of the rhi- noceros belongs to the skin, and is only an as- semblage of closely united hairs, but the horns of ruminants have a bony axis springing from the frontal bone. The bony prominence in the giraffe is covered by the skin permanently ; in the stags the bony core is at first under the skin, but soon becomes exposed, and after a certain time falls off, to give place to another similar growth; in the ox, sheep, goat, and antelope, the osseous axis grows during life, and never falls, being covered by a sheet of horn, growing by layers; these bony cores generally communicate with the frontal sinuses, and thus receive air into their interior. The species with falling horns have generally only the males thus armed ; the reindeer, however, is an exception. The comparative anatomy of the brain has been sufficiently given under the title BEAIN. Nervous System. The vertebrate nervous system is not homologous with the invertebrate ; the spinal cord of the former is enclosed in a vertebral canal, and its vesicular substance is continuous throughout, while the ganglionic chain of the latter is always in the general cavity with the viscera, forms a ring through which the oesophagus passes, and its vesicular substance is frequently interrupted. Though the vertebrate spinal cord cannot be considered as a chain of ganglia, it may be re- garded as a series of segments arranged in a linear direction, having a distinct enlargement in many animals at the origin of the spinal nerves, and particularly of those sent to the extremities. The spinal cord of fishes termi- nates near the end of the spine. Owen says that in typical fishes it gradually tapers to a point in the heterocercal or unequal-tailed spe- cies, but swells again into a terminal ganglion in most equal-tailed species ; in describing that of the angler (lophius), he has evidently fallen into an error in regard to its length, as shown by the researches of Prof. J. Wyman (see u Proceedings of the Boston Society of Natu- ral History," vol. iv., pp. 150, 151); the latter found the cord extending as usual quite near to the tail, where it ended in a ganglion, present- ing the striking peculiarity of being sheathed