DESCRIPTIONS OF PREPARATIONS. 



i. COMMON RAT (Mus decumanus), 



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 longi- 

 tudinal fissure in which is lodged the longitudinal sinus : next the cere- 

 bellum 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 oeso- 

 phagus, a 4owly vascular tube the small calibre of which is correlated with 

 the working of the dental apparatus in these creatures ; thirdly, trie third 



B 



