636 MAMMALIA. 



presternum with which in Monotremes an interclavicle is 

 fused, and with ivhich the clavicles (if well-developed) 

 articulate ; (b) a mesosternum divided into segments, with 

 ivhich the sternal parts of the ribs articulate ; and (c) a 

 xiphisternum, often cartilaginous. There are generally two 

 sacral vertebra, but to these several caudals, and more rarely 

 a lumbar, may be fused. The ilia slope downwards and 

 backwards, the ischia have no symphysis, but the pubes are 

 almost always united ventrally. 



The cerebral hemispheres have usually a convoluted surface, 

 and always cover the optic thalami and the optic lobes (now 

 four-fold corpora quadrigemina), and in higher forms the 

 cerebellum as well. The commissural system is well developed, 

 being especially represented by a large corpus callosum, except 

 in Monotremes and Marsupials, in which the anterior com- 

 missure is large and the corpus callosum small (according to 

 some), or absent (according to others). There is also an 

 important set of longitudinal fibres called the for nix. 



Except in Monotremes, in which there is a cloaca, the food 

 canal ends separately from the urinogenital aperture. 



The heart is four-chambered, and the temperature of the 

 blood is high, though less than that of Birds. There is but 

 one aortic trunk, which curves over the left bronchus. The 

 red blood corpuscles are, when fully formed, non-nucleated, and 

 are circular in outline, except in the Camelida where they are 

 oval. 



The lungs are invested by pleural sacs, and lie freely in 

 the chest cavity. Within the lungs the bronchial tubes fork 

 repeatedly into finer and finer branches. At the top of the 

 trachea there is a complex larynx with the vocal cords. 



The kidneys are generally compact and rounded bodies ; 

 the ureters open into the bladder, except in Monotremes in 

 which they enter a urinogenital sinus. Except in Monotremes, 

 the outlet or urethra of the bladder unites in the male with 

 the genital duct, to form a urinogenital canal ; in the female, 

 except in Monotremes and a few other cases, the urethra and 

 the genital duct open into a common vestibule. 



In the more primitive mammals the testes lie in the abdomen ; 

 in the majority they descend permanently (in a few cases 

 temporarily) into a single or paired scrotal sac, lying, except 

 in Marsupials, behind the penis. 



