superficial temporal 



superior sagittal sinus 

 I sinus rectus 



superficial facial vein 



transverse sinus 



occipital 



posterior auricular 

 postglenoid stem 



posterior cerebral 



internal carotid 



deep facial vein 



external maxillary (anterior facia 



internal maxillary 



laryngeal 

 external jugular 

 Figure 11-32. Veins of the head in the dog. (After Lounay, 1896) 



occipital artery 

 internal jugular 

 common carotid 

 external carotid 

 thyroid 



ventrolateral to the otic capsule, outside the ganglion of the 

 seventh nerve, and inside (below) the ganglion and root of 

 the fifth nerve. A vein following the course of the mandibu- 

 lar branch of the trigeminal enters the main channel here. 

 This branch has ma.xillary and mandibular divisions. The 

 main channel extends from here to the region behind the 

 eye where it receives infraorbital, orbital, and supraorbital 

 branches. The latter has a branch, the anterior cerebral 

 vein, which passes upward alongside the diencephalon to 

 the area above the brain, then forward over the telencepha- 

 lon near the midline. Below the brain there is a small con- 

 nective between the orbital sinuses of either side which passes 

 behind the tip of the infundibulum. 



In later stages, modification of the brain drainage occurs 

 (Figure 11-33). Involved are three main plexuses of vessels 

 enclosing the brain. The anterior plexus encloses the telen- 

 and diencephalon; its stem is the anterior cerebral vein. 

 Behind the ganglion of the fifth nerve is a stem for a plexus 

 enclosing the midbrain and the metencephalon; this is the 

 middle cerebral vein. The plexus of the myelencephalon 

 (medulla) is drained by the posterior cerebral vein passing 

 down, and just behind, the vagus nerve. 



As development proceeds, the dorsal ends of the anterior 

 cerebral channels join across the midline to form the sagit- 

 tal sinus. The three cerebral plexuses now become connected 

 by cross channels lying inside the cranial wall. The most 

 posterior cross connection joins the middle and posterior 

 stems above the otic capsule; this is the sigmoid sinus. The 

 middle cerebral stem below this connection now becomes the 

 superior petrosal sinus; above, it becomes the transverse 

 sinus which joins through a connecting channel with the 

 sagittal sinus of the anterior plexus. The anterior cerebral 

 stem now disappears. 



With the development of the cranial wall, the venous 



drainage is divided into interior (vena capitis medialis) and 

 exterior (vena capitis lateralis) channels connected through 

 the various foramina. The ophthalmic veins now enter the 

 cranium through the orbital fissure to reach the basilar 

 system which posteriorly drains through the internal jugular 

 stem. The basilar system (the vena capitis medialis) involves 

 the appearance of a new channel, the inferior petrosal sinus. 



A part of the vena capitis lateralis, now the internal 

 maxillary vein, drains the maxillary and mandibular veins 

 through a new channel below the middle-ear cavity into 

 the internal jugular stem; this new connective is the external 

 jugular. This evolution of channels is indicated among the 

 adults of mammals (Figure 11-34). The monotremes have 

 retained the primitive vena capitis lateralis channel passing 

 above the stapes. This channel, however, drains the cranium 

 through the anterior lacerate fissure. Marsupials vary; in 

 the opossum, the main cranial drainage is through the mid- 

 dle cerebral channel which enters an external jugular shunt 

 passing below the middle-ear cavity to reach the internal 

 jugular stem. In the foetus of Tnchosurtis, there is a typical 

 vena capitis lateralis. 



The inferior jugular of the embryo forms the lingual vein 

 of the adult; sometimes this becomes quite separate as an 

 anterior jugular. 



In the region of the heart, the anterior cardinals are 

 joined by a cross connective, the brachioradialis vessel, with 

 the result that blood from the left passes through this con- 

 nective to the right anterior cardinal to reach the heart. The 

 right common cardinal stem becomes the anterior vena cava. 



Among adult mammals this change in the anterior car- 

 dinal relationship can also be observed. In the monotremes 

 and marsupials, both anterior cardinal stems are retained 

 and enter the heart separately through their ducts of Cuvier. 

 In the monotremes, an innominate connective joins these 



362 . THE CIRCULATORY SYSTEM 



