Common Rat. 169 



Points of less classificatory importance are furnisbed to us by 

 the presence of a vena cava descendens on the left side, of smooth 

 cerebral hemispheres (Ji)-, of a uterus all but completely bifid (j/ 

 and /), of a Harderian gland (/), of a hibernating gland {k), 

 and of a double lacrymal gland {g and/). 



The left halves of the parietes of the craniospinal, thoracic, 

 abdominal, and pelvic cavities were removed to expose to view 

 the parts shown in this figure. The integument has been re- 

 moved from the greater part of the facial region, but a narrow 

 strip has been left connecting the concha of the ear with the 

 upper eyelid. A similar strip has been drawn as left in situ 

 overlying the costal attachment of the diaphragm. 



a. Left eye. 



b. Left ear. Both eye and ear are largely developed in most 



RodentSj and markedly so in this family. 



c. Diaphragm forming a contractile dome-shaped floor between 



the abdominal cavity below and the thoracic above. 



d. Eleventh dorsal vertebra, coimting from before backwards. The 



ten anterior dorsal vertebrae have their neural spines directed 

 more or less obliquely backwards ; the two vertebrae — one 

 only of which is seen in the figure — immediately behind d, 

 have their spines pointing forwards. The spine of the 

 eleventh dorsal vertebra has a vertical direction, and it 

 marks the point of greatest mobility between the segments 

 of the vertebral column, as also the point whence the size 

 of the vertebrae progressively increases both towards the 

 cervical and towards the sacral regions. The spine of the 

 second dorsal vertebra is of great length, and in the natural 

 condition of the parts it had an ossicle articulated to it, 

 homologous to the Ugamentum nuchae of longer-necked ani- 

 mals, and running forwards amongst the muscles of the 

 nape of the neck. The dorsal vertebrae are thirteen in 

 number, as is very commonly the case in this order, as 

 also among the Ruminantia and Carnivora. The first two 

 cervical vertebrae are, as is usual, though not invariable, 

 among mammals, much larger than any of the five which 

 succeed them in the neck. 



e. Spinal cord. The part where it widens into the medulla ob- 



longata, a part of considerable transverse dimensions in 



