MAMMALIA. 



235 



vena cava : they never anastomose with the me- 

 senteric veins. 



The kidneys are relatively smaller, and 

 present a more compact figure than in the 

 other vertebrate classes ; their parenchyma 

 is divided into a cortical and medullary por- 

 tion, and the secerning tubuli terminate in 

 a dilatation of the excretory duct called the 

 pelvis. The tubuli uriniferi are slightly 

 branched, and the ramification takes place in 

 the dichotomous, and not pinnatifid manner: 

 they are convoluted in the cortical, and straight 

 in the medullary portions of the kidney, and 

 with a few exceptions terminate upon valvular 

 prominences, called mammillve. The ureters 

 convey the urine to a urinary bladder situated 

 anterior to the rectum, and to the genital tubes 

 or cavities. The liver is generally divided into 

 a greater number of lobes than in birds. The 

 portal system is formed by veins derived ex- 

 clusively from the spleen and chylopoietic 

 viscera. The cystic duct, when it exists, 

 always joins the hepatic, and does not enter 

 the duodenum separately. The pancreatic duct 

 is commonly single. 



The mouth is closed by soft flexible mus- 

 cular lips. The upper jaw is composed of 

 palatine, maxillary, and intermaxillary bones, 

 and is fixed ; the lower jaw consists of two 

 rami, which are simple, or formed by one 

 bony piece, and are articulated by a convex or 

 flat condyle to the base of the zygomatic pro- 

 cess, and not to the tympanic element of the 

 temporal bone ; the base of the coronoid pro- 

 cess generally extends along the space between 

 the condyloid and the alveolar processes. The 

 jaws of Mammals, with few exceptions, are 

 provided with teeth, which are arranged in a 

 single row ; they are always lodged in sockets, 

 and never anchylosed with the substance of 

 the jaw ; in most cases they present dif- 

 ferent forms in the same individual, and the 

 molars have two or more fangs. Never more 

 than two teeth succeed each other in the ver- 

 tical direction, and in this case the fang of the 

 deciduous tooth is always completed before it is 

 shed. The tongue is fleshy, well developed, 

 with the apex more or less free. The posterior 

 nares are protected by a soft palate, and the 

 larynx by an epiglottis ; the rings of the trachea 

 are generally cartilaginous and incomplete be- 

 hind : there is no inferior larynx. The oeso- 

 phagus is continued without partial dilatations 

 to the stomach, which varies in its structure 

 according to the nature of the food, or the 

 quantity of nutriment to be extracted therefrom. 

 An epiploon of greater or less extent is conti- 

 nued from the great curvature of the stomach. 

 The termination of the duodenum is generally 

 tied closely to the spine, above the root of the 

 mesentery. The colon is suspended generally 

 by a distinct duplicature of peritoneum, called 

 the meso-colon : the caecum coli when present is 

 usually single. The rectum commonly termi- 

 nates by an aperture distinct from that of the 

 urinary or genital canals. 



The female generative organs consist of two 

 ovaries, which with very few exceptions are 

 equally developed ; there are always two ovi- 



ducts or Fallopian tubes, a simple or more or 

 less completely bifid uterus, a vagina, which 

 is commonly single, and a clitoris. The es- 

 sential element of the ovum, the germinal vesi- 

 cle, acquires a surrounding granular stratum 

 (tunica granulosa), a small vitelline mass, and 

 a vitelline membrane, before it quits the ovisac. 

 The ovisacs or Graarian vesicles, consisting of 

 an ovarian vesicle and a vascular layer of the 

 condensed cellular tissue, or stroma of the ovary, 

 are never pendent, and rarely racemose. 



The male organs consist of two equally de- 

 veloped testes, commonly situated in an ex- 

 ternal tegumentary pouch or scrotum : the vasa 

 deferentia form an epiclidymis at their com- 

 mencement, and frequently communicate with 

 the ducts of vesiculae seminales at their termi- 

 nation, and the semen is conveyed outwardly 

 along a complete urethral canal, where it is 

 mingled with the secretion of certain accessory 

 glands, of which those called " Cowperian," or 

 " preprostatic," are constant ; there are also ge- 

 nerally prostatic and in many cases vesicular 

 glands. The penis is always perforated by an 

 urethral canal, along which, with very few ex- 

 ceptions, the urine as well as the semen is con- 

 ducted from the body. 



The true vertebrae of Mammalia have their 

 bodies ossified from three centres, and present 

 for a longer or shorter period of life a com- 

 pressed epiphysis at each extremity. They are 

 articulated by concentric ligaments with inter- 

 posed glairy fluid, forming what are called the 

 intervertebral substances ; the articulating sur- 

 faces are generally flattened, but sometimes, as 

 in the neck of certain Ruminants, they are 

 concave at one end and convex at the other; 

 such a vertebra, however, may be distinguished 

 from a vertebra of a Reptile, with a similar 

 ball-and-socket structure of the articular sur- 

 faces, even when found in a fossil slate, and 

 when the test of the articulating medium can- 

 not be applied, by the constant anchylosis or 

 confluence of the annular with the central part 

 or body. The cervical vertebrae, with one or 

 two exceptions, are seven in number, neither 

 more nor less ; the Monotremes, which are the 

 instances commonly opposed to other generaliza- 

 tions, form no exception to this rule. The 

 lumbar vertebrae are more constant and more 

 numerous than in other classes of vertebrate 

 animals. The atlas is articulated by concave 

 articular processes to two convex condyles, 

 which are developed from the ex-occipital ele- 

 ments of the last cranial vertebra. The tym- 

 panic element of the temporal bone is re- 

 stricted in function to a subserviency to the 

 organ of hearing, and never enters into the 

 articulation of the lower jaw. The frontal 

 bones are developed each from a single centre ; 

 there are no anterior or posterior frontals. The 

 olfactory nerves escape from the cranial cavity 

 through numerous foramina of a cribriform 

 plate. The optic foramina are always distinct 

 from one another, and generally from the fis- 

 sure lacerae anteriores, and consequently give 

 passage only to the optic nerves and ophthalmic 

 arteries. The carotid canals do not intercom- 

 municate. The cranial bones, in regard to the 



