408 THE ANATOMY OF VEETEBEATED ANIMALS. 



inferior is long and wide, but is not especially dilated near 

 tlie heart. Muscular fibres are not continued on to it from 

 tbe diaphragm. The aorta and pulmonary arteries are not 

 dilated at their origins. The arteries have a great tendency 

 to break up into plexuses. Thus the internal carotids 

 form gi'cat networks which communicate with vertebral 

 plexuses, extending throughout the entire spinal canal. 

 The brachial artery divides into two branches, and these 

 subdivide into innumerable parallel twigs. The intercostal 

 arteries are the chief source of the large thoracic plexuses, 

 which lie at the sides of the vertebral column in the 

 dorsal half of the thorax. Finally, an arterial rete mirabile 

 suiTounds the caudal aoi-ta. The veins form plexuses cor- 

 responding to, and mixed up with, those of the arteries ; 

 and a very large venous plexus lies on the subvertebral 

 muscles in the abdomen and thorax. 



The respu'atory apparatus of the Porpoise presents many 

 remarkable peculiarities. The contour of the fi'ont part 

 of the head, as bounded by the integument, is very convex — 

 the corresponding facial region of the skull, on the con- 

 trary, is very concave. The interval between the two is 

 occupied, in part, by fibrous and fatty tissue; and, in part, by 

 a singiilarly sacculated spiracular chamber, which connects 

 the single spiracle with the double external nares of the skidl. 

 Two valves, an anterior and a posterior, lie immediately 

 above these external nares and close the communication 

 between them and the chamber, except at such times as it 

 is forced open from below. Bach nasal passage remains 

 distinct from the other as far as the valves, the middle of 

 each of the latter being fastened to the septiim, so that there 

 may be said to be a pair of valves for each opening between 

 the passages and the spiracular chamber. Each nasal pass- 

 age, after it ceases to be surrounded by bone, sends off two 

 diverticula, one forwards and one backwards. The anterior, 

 which lies between the anterior valve and the premaxilla, 

 is a simple sac, lined with a thin, black, smooth membrane. 

 The posterior diverticulum lies between the posterior valve 

 and the ethmoid and nasal bones. It is incompletely 



