efferent pseudobronchial ortery 

 internal carotii 



afferent pseudobronchial artery 



ophtholm 



external ma 



anterior efferent hyoid 

 Figure 11-30. Anterior arteries of Ch/omydose/cchus. (After Allis, 1923) 



lateral to the infundibulum. Here the internal carotids 

 penetrate the cranial wall and enclose the infundibulum and 

 pituitary in an interconnecting network of arteries. The two 

 stems send branches to the eye muscles and also give rise to 

 anterior and middle cerebrals. 



The main stem now passes across the ventroanterior wall 

 of the orbit and gives off a large palatine artery to the roof 

 of the mouth cavity, the anterior end of the tongue muscles, 

 and the ventral wall of the buccal funnel. The main stem 

 passes out through the anterior inner margin of the orbit, 

 along with the large supraorbital nerve trunk, and extends to 

 the dorsal and lateral margins of the buccal funnel. This or- 

 bitonasal stem has branches to the nasal area and to the 

 musculature anteroventral to the eye. 



Myxine is much like the lamprey. The lingual artery, 

 which may represent the external carotid, is a branch of 

 the internal carotid. The paired internal carotids are ac- 

 companied for some distance by a median vessel: they con- 

 tinue forward beneath the brain capsule and out to the upper 

 lip. Branches from these vessels serve the various parts of 

 the head. 



Genera/ observations on head arteries The pattern of head 

 arteries in the tetrapods shows many minor variations, 

 variations traceable to the strong specialization shown by 

 some members, the frog for example. Among the fishes, the 

 tetrapod pattern is scarcely recognizable although there is 

 some similarity which is reflected in the use of common 

 terms. The development of a pseudobranchial circulation in 

 the shark and actinopterygian suggests that this was com- 

 mon to early fishes and has been lost both in the choanate 

 and the holocephalan. Since the pattern of the agnath is quite 

 remote, little direct comparison with other fishes is possible. 

 Lack of a pseudobranchial circulation can be mentioned 

 and the system might be viewed as generally less modified 



than in gnathostomes, or, perhaps, it might better be de- 

 scribed as independently evolved. 



Veins 



Mammals The main veins of the head of the mammal 

 can be described in terms of the human (Figure 11-31). 

 The anterior (superior) vena cava branches to form right 

 and left internal jugulars and subclavians. The internal 

 jugular of either side passes up next to the vertebral centra. 

 Behind the jaw it enters the skull at the jugular foramen to 

 receive the veins from the brain. 



The superior sagittal sinus lies in the base of the mem- 

 branous falx which extends down between the cerebral lobes. 

 Bilateral great cerebral veins exit from the ventricles of the 

 cerebrum and join with the inferior sagittal sinus, which 

 extends back along the ventral margin of the falx, to form 

 the straight sinus. This midline channel joins posteriorly 

 with the superior sagittal sinus (the confluence of the sinuses). 

 The sagittal sinus now leads into bilateral transverse sinuses 

 which follow the base of the tentorium. Then an occipital 

 sinus continues along the midline to the small vessels of 

 the region of the foramen magnum. The transverse sinuses 

 laterally become the sigmoid sinuses, which in turn become 

 the internal Jugulars with passage through the jugular 

 foramina. Superior and inferior petrosal sinuses connect 

 the sigmoid sinus of either side with the basilar plexus in 

 the region behind the sella turcica. 



The internal jugular receives a number of vessels from 

 the superficial regions of the head. An occipital vein, which 

 passes up behind the skull, connects, through the skull, with 

 the transverse sinus or the confluence of the sinuses. There 

 is also a connective between the occipital vein and the mas- 

 toid division of the posterior auricular vein. From the in- 

 ternal jugular, below the jaw, there are several branches — 

 lingual, pharyngeal, and thyroid. 



360 • THE CIRCULATORY SYSTEM 



