236 



MAMMALIA. 



number and persistency of their sutures, are 

 intermediate in character to those of Birds and 

 Reptiles. The anterior extremity of the skull 

 is always formed by both the upper and lower 

 jaws. The scapula is generally an expanded 

 plate of bone; the coracoid, with two excep- 

 tions, appears as a small process of the scapula : 

 the clavicle is inconstant as to its presence ; the 

 sternum consists of a narrow and usually simple 

 series of bones; the sternal portions of the ribs 

 are generally cartilaginous, and fixed to the 

 vertebral portions without the interposition of 

 a distinct articulation ; there are no abdominal 

 ribs or abdominal sternum. The pubic and 

 ischial arches are generally complete, and 

 united together by bony confluence on the 

 sternal aspect, so that the interspace of the two 

 bony pelvic arches is converted into two holes, 

 called foramina obturatoria, or thyroidea. 



The brain in Mammalia consists of cerebral 

 and optic lobes, cerebellum and medulla ob- 

 longata, but the optic lobes are placed on the 

 upper part of the crura cerebri, are solid, are 

 always divided by a transverse fissure, and are 

 generally concealed by the cerebral lobes, in 

 consequence of the large relative size of these 

 masses; the decussation of the corpora pyrami- 

 dalia is always distinctly marked: the tuber 

 annulare and lateral lobes of the cerebellum 

 have a correspondingly conspicuous develop- 

 ment; the ventricles of the cerebrum are large, 

 and contain a corpus striatum and cornu am- 

 inonis ; the fornix is always well developed, 

 but the following aphorisms in Cuvier's cele- 

 brated abstract or condensation of M. Serres' 

 prize essay are applicable only to the placental 

 Mammalia, viz. " The corpus callosurn as well 

 as the pons Varolii are peculiar to Mammalia. 

 The corpus callosum is developed in direct 

 proportion to the size of the corpora striata 

 and hemispheres. It increases progressively 

 from Rodentia to Man. The corpus collosum 

 is developed in direct proportion to the de- 

 velopment of the tuber annulare." 



In the Marsupial Order, as in the Kangaroo 

 and Wombat, the cerebral hemispheres are 

 relatively larger and more complicated with 

 convolutions than in any Rodent, yet the trans- 

 verse commissure which represents the rudi- 

 ment of the corpus callosum connects only the 

 hippocampi majores ; it is not separated from 

 the fornix by any septum lucidum ; and, upon 

 divaricating the cerebral hemispheres, the la- 

 teral ventricles are as much exposed as when, 

 in placental Mammalia, the corpus callosum 

 has been removed. The tuber annulare, how- 

 ever, exists in the Marsupial as in the Placental 

 Mammalia, and illustrates its correlation with 

 the lateral lobes of the cerebellum. The class 

 Mammalia is the only one in which the cere- 

 bral hemispheres are observed to have their 

 vascular superficies multiplied or increased by 

 convolutions, which arrive at their maximum 

 of development in Man. 



The olfactory nerves of mammals are soft, 

 and divide into numerous branches in the 

 cranium, which pass out by the orifices of the 

 cribriform plate of the aethmoid. 



The nervi vagi principally supply the larynx, 



form a plexus around the oesophagus, and do 

 not unite into a single trunk before passing to 

 the stomach. The left recurrent nerve, and not 

 the right, bends round the trunk of the aorta. 



The cervical portion of the sympathetic nerve 

 passes along the neck on the sternal aspect of 

 the transverse processes of the vertebra ; and 

 its trunk in the thorax and abdomen is not 

 immediately connected with the ganglia of the 

 spinal nerves. The splanchnic nerves form 

 large ganglia before giving off the visceral 

 plexuses. 



The sclerotic coat of the eye is a fibrous 

 membrane, and never contains bony plates. 

 In the quantity of aqueous humour and the 

 convexity of the lens, Mammals are generally 

 intermediate to Birds and Fishes ; but they 

 have no marsupium or pecten, nor any cho- 

 roid gland. 



The organ of hearing is characterized in 

 Mammalia by the full development of the 

 cochlea with a lamina spiralis ; there are three 

 distinct ossicles in the tympanum ; the mem- 

 brana tympani is generally concave externally, 

 and the meatus auditorius externus often com- 

 mences with a complicated external ear, having 

 a distinct cartilaginous basis. 



The external apertures of the organ of smell 

 are provided in Mammalia with moveable 

 cartilages and muscles, and the extent of the 

 internal organ is increased by accessary cavities 

 or sinuses which communicate with the pas- 

 sages including the turbinated bones. 



The tongue is always soft and fleshy, and its 

 gustatory surface is provided with conical, fos- 

 sulate, and fungiform papilla ; it is supplied 

 by a large proportion of the third division of 

 the fifth pair of nerves, as well as by the ninth 

 and glosso-pharyngeal. 



Classification. The Mammalia were first 

 separated from other four-footed animals and 

 distinguished as a class or particular group 

 (genos) by Aristotle, the founder of natural 

 history, by whom they were denominated 

 .Zootoka, or Viviparous animals. The Greek 

 philosopher divided the Zootoka, according to 

 the nature of their locomotive organs, into 

 three sections : 1st, Dipoda, or bipeds ; '2d, 

 Tetrapoda, or quadrupeds ; and 3d, Apoda, 

 or impeds, which comprehends the Whale- 

 tribe. The second of these primary divisions, 

 the quadrupeds, which includes by far the 

 largest proportion of the class, and in common 

 parlance is considered as the class itself, is 

 subdivided by Aristotle into two great natural 

 groups, according to the modifications of the 

 organs of touch. In the first of these groups, 

 the extremities of the digits are left free for 

 the exercise of the tactile sense, the nail or 

 claw being placed on one side only (Ungui- 

 culata of Ray) ; in the second group, the 

 digits are inclosed in hoofs (Ungulata of Ray). 

 For the convenience of treating of the different 

 forms of the Unguiculate quadrupeds, Aristotle 

 employs for their further subdivision another 

 system of organs, viz. the teeth. His first group 

 or family is composed of those Unguiculates 

 which have the front teeth trenchant, or termi- 

 nating in a cutting edge, and the back teeth 



