Cranial Thyroid Artery Research Articles

‎“TOPOGRAPHY OF THE THYROID GLAND, ITS BLOOD SUPPLY AND HISTOLOGICAL STRUCTURE IN ONE ‎HUMPED CAMEL” (CAMELUS DROMEDERIUS).‎

The thyroid gland of Camelus dromedrius is located directly behind the larynx. It consists of two lateral lobes at the sides of trachea. The lobes are connected to each other by a caudal isthmus from the ventral surface. The lobes extend to the seventh tracheal ring and even to the eighth or ninth ring. The medial surface of the lobes is adhered to trachea. The lateral surfaces are convex. Also, the dorsal and ventral edges of the lobes are convex. The thyroid gland receives blood via caudal thyroid artery and cranial thyroid artery. A third artery which is initiated from the common carotid artery serves as a medial thyroid artery. Histological observation showed that the lobes and isthmus are surrounded by an external capsule mainly of loose connective tissue and an internal capsule of dense irregular connective tissue. The follicles of the gland sometimes appear as regular follicles but other follicles are of semi tubular shape or branched.

Morphometry, topography and arterial supply of the thyroid gland in rhesus monkeys (Macaca mulatta Zimmermann, 1780).

The purpose of this research was to investigate the measures, topography, and vascularization of the thyroid gland in Macaca mulatta, a non-human primate. The study involved the dissection of ten male adult cadavers of Macaca mulatta. The length, width, and thickness of the right lobe of the thyroid were 2.552 ± 0.341, 1.019 ± 0.137, and 0.729 ± 0.137 cm. These measures in the left thyroid lobe were 2.406 ± 0.299, 1.013. ± 0.087, and 0.769 ± 0.083 cm. The study found no significant differences in the measures of the left and right lobes of the thyroid gland in rhesus monkeys. Regarding topography, the thyroid gland was located ventrolateral to the trachea, similar to its position in other mammal species. The cranial pole of the gland was closely related to the cricoid or thyroid cartilage, while the caudal pole showed variable positioning to the tracheal rings. The isthmus, a thin band of tissue connecting the lobes, was present in all specimens. The cranial thyroid artery was found to originate from the external carotid artery in most specimens. It supplied the thyroid gland and sent branches to muscles in the neck region. The caudal thyroid artery, originating from the common carotid artery, provides additional blood supply to the gland and sends a branch to the esophagus. This research contributes to knowledge about the thyroid gland in non-human primates, specifically Macaca mulatta. The findings provide critical information for comparative studies and understanding the thyroid gland's role in health and disease.

THE GROSS AND MICROSCOPIC ANATOMY OF THE CAROTID BODY OF AN ENDOGENOUS DUCK

The carotid body in an endogenous duck was closely related to the ventral surface of the common carotid bifurcation zone caudal to the parathyroid glands. The carotid body was characterized by its compact and fusiform shape. Taking into account all carotid bodies, the mean length was 1.058 +0.95 micrometers. The blood supply of the carotid body in duck was not detected easily but caudal and cranial thyroid arteries gave many branches to it. Histologically, the carotid body of an endogenous duck was consisted of two types of cells; chief cells which occurred in groups and were invested by sustentacular cells. These cells were embedded within a connective tissue fibers to form glomic tissue. A collection of ganglionic cells were observed in the vicinity of the carotid body

SOME TOPOGRAPHICAL AND HISTOLOGICAL STUDIES OF THYROID AND PARATHYROD GLANDS OF MOORHEN (Gallinula c. chloropus)

The thyroid gland of Moorhen has two separated lobes. These lobes were located in the throracic inlet and receive blood supply from the cranial, middle and caudal thyroid arteries.
 The histological organization of the thyroid gland in Moorhen is surrounded by a distinct connective tissue capsule and the parenchymal cells were arranged into colloid filled follicles enmeshed in the highly vascular interstitial connective tissue. The bilaterally paired, round to oval, parathyroid glands in Moorhen were located intrathoracically near or close to the caudal pole of the thyroid glands. They receive blood by short branches from caudal thyroid artery and small branch from the common carotid artery.
 The parathyroid glands in Moorhen have a thin connective tissue capsule. Its parenchymal cells were arranged into an irregular, anastomosing cords of chief cells. No oxyphil cells were found in the parathyroid glands of Moorhen.

Ultrasonographic features of the normal thyroid gland in adult horses

ABSTRACT: Few reports have been published regarding the use of ultrasonography as a method of evaluating the normal thyroid gland in horses. For these reasons, this study aimed at determining reliably of the thyroid measurements from the comparison between the left and right thyroid lobes, as well as assessing the contour, format, echotexture and echogenicity of the healthy thyroid by mode-B ultrasonography. Additionally, the equine thyroid vascularization was quali-quantitatively characterized the by Doppler. The sample size initially was determined by the animal selection with advanced age and without volume increase in the neck proximal region. Finally, eleven horses were selected by laboratory test, search of thyroid neoformations by ultrasonography and cytology thyroid. Next, these animals were submitted to thyroid lobes ultrasonographic evaluation. Excellent reproducibility was observed for all measurements obtained. Among the comparisons made between the quantitative parameters of the left and right lobes, it was observed that there was only difference between their respective lengths. Qualitatively, there was a significant variation between the lobes elliptical format in the longitudinal plane of some glands, which oscillated between a rounded and flattened conformation. Thus, we can conclude that the difference between the lobes format of some thyroids can be explained by the significant difference observed between the length of the left and right lobes. Additionally, it was verified that there was no difference between the Doppler quantitative parameters. Therefore, we may suggest that unilateral analysis of the cranial thyroid artery by spectral Doppler can be used to evaluate equine thyroid diffuse disorders.

Slc:Wistar/ST rats develop unilateral thyroid dysgenesis: A novel animal model of thyroid hemiagenesis

Developmental anomalies of the thyroid gland lead to congenital malformations such as thyroglossal duct cysts and thyroid dysgenesis. However, the pathogenesis of thyroid dysgenesis remains unclear due to the lack of suitable animal models. This study demonstrated that Slc:Wistar/ST rats frequently developed unilateral thyroid dysgenesis, including hemiagenesis, characterized by the absence of one lobe. In Wistar/ST rats, each thyroid lobe was frequently different in size, and approximately 27% and 20% of the rats presented with hemihypoplasia and hemiagenesis of the thyroid gland, respectively. Dysgenesis was predominant on the left side in both sexes, without sex differences. At a young age, thyroid hemiagenesis did not alter body weight. In rats of both sexes with thyroid hemiagenesis, plasma total triiodothyronine and total triiodothyronine levels remained unchanged while plasma thyroid-stimulating hormone levels were significantly elevated in young rats. The remaining thyroid lobes increased in weight, but the follicular epithelial cells appeared normal in terms of their height and proliferating activities. On the side of thyroid dysgenesis, the parathyroid glands were normally localized and were situated at the same location as the contralateral glands. The ultimobranchial body remnants were localized at the level of the thyroid gland along with the cranial thyroid artery and vein, forming cell clusters or cystic structures and containing calcitonin-positive C-cells. In conclusion, Wistar/ST rats developed unilateral thyroid dysgenesis and may be novel and useful animal models for thyroid hemiagenesis in humans and for morphogenesis of pharyngeal pouch-derived organs.

Restricted localization of ultimobranchial body remnants and parafollicular cells in the one-humped camel (Camelus dromedarius).

Parafollicular cells (C-cells) exist within the thyroid glands and display different distributions within the glands among mammalian species. In the one-humped camel (Camelus dromedarius), localization of the C-cells remains under debate. We herein investigated appearance of C-cells and the remnants of the ultimobranchial body, origin of C-cells, in the thyroid glands of one-humped camels. Macroscopically, a white mass was present at one-third the length from the cranial end of the thyroid glands where the cranial thyroid artery entered. In addition, large fossae were frequently found adjacent to the white mass. Histologically, the mass was mainly composed of connective tissues, thyroid follicles, and two types of cell clusters: one was composed of cells with clear cytoplasm and the other was composed of non-keratinized epidermoid cells. The mass and the fossae contained p63-positive cells, indicating that they consisted of ultimobranchial body remnants. Calcitonin was expressed in cells with clear cytoplasm, which were localized just beneath the fossae and in the cell clusters of the white mass. C-cells also resided in both subfollicular and interfollicular spaces adjacent to the white mass, but gradually decreased toward the periphery. C-cells tended to display round shapes in the ultimobranchial body remnants and subfollicular spaces, and spindle shapes in interfollicular spaces. In conclusion, we demonstrated that the ultimobranchial body remnants were limited to the region around the entrance of cranial thyroid artery and vein, and C-cells were mainly concentrated within and around the ultimobranchial body remnants.

Biomechanical study on decellularized laryngeal scaffold in dogs

To evaluate the biomechanical characteristics of the decellularized laryngeal scaffold. Ten Chinese adult dogs were randomly divided into two groups: perfusion group (n = 5) and control group (n = 5). The acellular larynx scaffold was obtained from dogs through cranial thyroid arteries perfusion with detergents. Comparative examinations were performed by the macroscopic view, histological view (hematoxylin and eosin stain, Alcian blue stain and Masson stain), scanning electron microscope (SEM) and biomechanical properties between perfusion group and control group. Macroscopic view showed that the decellularized laryngeal scaffold appeared pale asphyxia. HE stain indicated that there were little acellular traces of muscle and mucosa. Alcian blue stain, Masson stain and scanning electron microscope (SEM) suggested that there were no obvious changes about glycosaminoglycan and collagen. The compressive modulus of thyroid cartilage was (1.06 ± 0.07) MPa (x(-) ± s) in experimental groups and (1.15 ± 0.11) MPa in control group, showing no significant difference (t = 1.424, P > 0.05), neither in compressive modulus of annular cartilage (1.68 ± 0.11) MPa in experimental groups and (1.67 ± 0.09) MPa in control group (t = 0.185, P > 0.05). The tensile strength of thyroid cartilage between experimental (5.74 ± 0.88) MPa and control groups (6.18 ± 1.33) MPa did not have the statistical significance (t = 0.627, P > 0.05). These results indicate that perfusion method can construct a perfect biomechanical acellular larynx scaffold which could be a better selection for laryngeal reconstruction with tissue engineering method.

Acute severe hemorrhage secondary to arterial invasion in a dog with thyroid carcinoma

A 7-year-old spayed female Labrador Retriever was referred because of progressive swelling and bruising of the neck, hemothorax, a possible mediastinal mass, and stridor. At the time of admission, the dog was recumbent, mentally dull, tachycardic, and hypothermic. Fluid therapy with hydroxyethyl starch and a balanced electrolyte solution was administered because of presumed hypovolemic shock secondary to hemorrhage; multiple units of packed RBCs and fresh frozen plasma were also administered. On the basis of the computed tomographic images, extensive subcutaneous, subfascial, and cranial mediastinal hemorrhage; hemothorax; prescapular lymphadenopathy; and a contrast-enhancing mass in the left cranioventral aspect of the neck were diagnosed. Exploratory surgery of the neck was performed. All subcutaneous structures were encased in a large blood clot. During dissection of the clot, pulsatile bleeding was observed just caudal to and to the left of the larynx; ligation of the left common carotid artery resulted in immediate cessation of the pulsatile bleeding. Further surgical exploration revealed a 2.5 x 2.5-cm mass adhered to the left common carotid artery at the level of the branching to the left cranial thyroid artery. The mass was removed; the histologic diagnosis was thyroid carcinoma. Three weeks after surgery, treatment with a combination of radiation and chemotherapy (doxorubicin and carboplatin) was begun. Thirteen months after surgery, the dog continued to be free from clinical signs of disease.

Microvascular transfer of long tracheal autograft segments in the canine model.

The reconstruction of long segment tracheal defects represents an unsolved clinical dilemma. Prior attempts to directly revascularize tracheal segments have been unsuccessful. The objective of this study was to evaluate orthotopic autotransplantation of revascularized long tracheal segments in the canine model. Ten randomly selected mongrel dogs underwent excision, orthotopic reimplantation, and microvascular revascularization of a long segment (8.0 cm) of cervical trachea. The cranial thyroid artery and the internal jugular vein served as the vascular supply for the tracheal segment. The animals were maintained for a period of 30 days during which time graft viability was measured by routine endoscopic assessment and tracheal biopsies. Ex vivo, tracheal autografts were examined grossly for graft healing an d microscopicallyfor histologic architecture. Seven of 10 dogs survived the predetermined 30-day postoperative study period without complications. Postmortem examination demonstrated that 7 dogs had healed tracheal autograft segments with normal histologic architecture, 2 dogs sustained a postoperative wound infection and tracheal dehiscence, and 1 dog sustained a fatal postoperative hematoma. For the first time, we have demonstrated direct revascularization of long segment tracheal autografts in the dog model using the cranial thyroid artery and internal jugular vein as the vascular supply.

Chemical control of laryngeal vascular resistance in anesthetized dogs.

The branch of the cranial superior thyroid artery that supplies the larynx was perfused unilaterally at a constant flow rate in dogs anesthetized with pentobarbitone sodium (30 mg.kg-1, i.v.). Laryngeal vascular resistance (RLv) was calculated. Intraluminal laryngeal pressure (PL) was determined. Asphyxia caused a diminution in PL (-27.7 +/- 4.2%) (p < 0.01) and an initial decrease followed by an increase in RLv (-10.9 +/- 2.3% and +12.8 + 2.8%) (p < 0.001). Following vagotomy, asphyxia decreased PL (-17.1 +/- 4.2%) and increased RLv (+45.5 +/- 11.0%). Systemic hypoxia induced by breathing 8% O2-N2 increased RLv (+8.4 +/- 1.3%) (p < 0.001) and decreased PL (-24.9 +/- 3.7%) (p < 0.001). In chemodenervated dogs, the response of PL to hypoxia was abolished while that of RLv was similar to that in intact animals. Breathing of 7% CO2 in air caused an increase in RLv (+6.1 +/- 1.6%) (p < 0.001) and a decrease in PL (-20.9 +/- 3.2%) (p < 0.001). Following chemodenervation, hypercapnia still increased RLv (+11.3 +/- 3.2%) (p < 0.01) and diminished PL (-29.9 +/- 2.5%). Bilateral vagotomy reduced the response of PL to hypercapnia (-13.3 +/- 6.9%) (p < 0.05). Local intra-arterial injection of KCN produced an increase in RLv and PL (+7.0 +/- 1.2%, +9.0 +/- 1.0%). After peripheral chemodenervation KCN injection slightly increased PL (+2.0 +/- 0.9%). The response of RLv was almost abolished. In conclusion, laryngeal vasoconstriction in response to systemic hypoxia after chemodenervation is probably mediated by increased sympathetic discharge to the larynx due to hypoxia of the CNS. Laryngeal responses to hypercapnia may be due to the action of central chemoreceptors.

Use of the peritracheal fold in the dog tracheal transplantation model.

To investigate the technical aspects of the canine model of human tracheal transplantation for potential application to reconstruction of extremely long tracheal defects (> 10 cm). In phase 1, long tracheal segments were skeletonized and pedicled with the thyroid glands, cranial thyroid arteries and veins, and internal jugular vein branches. The segments were elevated completely, attached to the vascular pedicle only, and replaced with primary tracheal anastomoses. In phase 2, long segments were elevated along with a diffuse soft tissue "blanket" that envelops the trachea and thyroid glands. Because this study was designed to primarily address, in situ, tracheal perfusion territories of a cranially located vascular pedicle, microvascular anastomoses were not conducted. Two small-bodied beagles (10-15 kg) and 5 large-bodied mixed-breed dogs (20-30 kg) were humanely killed 2 to 41 days after surgery, and anatomic and histological analyses were conducted. Unlike that of humans, the thyroid gland complex of dogs is not intimately associated with the trachea but is conjoined with a peritracheal soft tissue "fold." Within this fold, blood is transmitted to the trachea via a diffuse, segmental vascular plexus. In phase 1, pronounced tracheal necrosis occurred within 2 to 5 days. In phase 2, extremely long tracheal segments (10-12 cm), based only on a cranially located pedicle, were still viable at 2 to 6 weeks. Preservation of the "peritracheal fold" in the dog model of tracheal transplantation is critical to the onset and maintenance of vascular perfusion in a long tracheal segment. Furthermore, the use of large-bodied dogs is necessary to provide for a usable venous efflux component.

Tracheal transplantation: Defining the vascular territory of the canine cranial thyroid artery

The management of long-segment tracheal stenosis remains an unsolved clinical problem that has significant implications for the survival of some patients and the quality of life of others. However, recent advances in microvascular tissue transfer may allow tracheal transplantation to be used as a treatment option. Studies in dogs have demonstrated that the reanastomosis of the cranial thyroid artery (CTA) after the transplantation of a short segment of trachea sustains cartilage viability. However, the distal perfusion zone of the CTA within the canine trachea has not been described. Hence it is unclear what length of trachea can be safely harvested, with use of the CTA as the donor artery, without risking ischemia of the distal trachea. This study determined the perfusion territory of the CTA by use of latex or dye infusion via the thyroid branches of the CTA. Findings in five perfusion-fixed cadaver mongrel dogs (8 to 12 kg) were studied. In each dog, both common carotid arteries were dissected and the CTAs identified. The right CTA was ligated and the left common carotid artery was ligated cranial to the CTA. The left common carotid artery was perfused for 10 minutes under 100 mm Hg pressure with black india ink/saline solution (20:1000 ml) or with blue latex solution. Perfusion territories were determined by grossly and microscopically measuring the region of stain in the endolaryngeal and endotracheal airways. The results demonstrated that up to 18 ipsilateral rings of the canine trachea are perfused after injection of dye under physiologic pressure into the CTA. Contralateral staining was also noted, but to a lesser degree. These results suggest that the canine CTA may perfuse, on average, a 16- to 17-ring tracheal segment. (Otolaryngol Head Neck Surg 1999;120:180-3.)

Comparison of two types of arterialized venous forearm flaps for oral reconstruction and proposal of a reliable procedure

The mechanism of survival of venous flaps is not yet completely understood. To determine the conditions for reliable success, in 1996 and 1997 we used arterialized venous forearm flaps for oral reconstruction in patients with neoplastic disease. Building on earlier experience with flow-through flaps, we compared two different designs obeying the following specifications: five so-called arterialized flow-through flaps, elevated from the ulnar flexor side of the forearm, with a single vein for orthograde inflow and outflow, diameter not larger than 2 mm (type 1); and five arterialized flaps with two parallel veins on the proximal side of the flap, also with a diameter of 1-2 mm, to avoid by-passing of flap tissue by 'catching' the blood in the venous/capillary system (type 2). There were nine male and one female patients with a mean age of 55.7 years. We covered defects of the anterior or anterolateral floor of the mouth to avoid folding of the flaps. Microsurgical anastomosis was done to the facial artery, the cranial thyroid artery and the external jugular vein or the facial vein. Average size of the flaps was 23 +/- 7.28 cm2 (type 1) and 25 +/- 5 cm2 (type 2). Four flaps of type 1 showed total or subtotal success and only one was lost because of a haematoma. In comparison with this we saw three losses in type 2 and only one partial and one total success, respectively. Analysis showed regularly occurring problems: there was failure of venous drainage in the lost type 2 flaps one day after microsurgery. In the flow-through flaps and the surviving type 2 flaps which had visible communicating branches between the two veins, the partial by-passing of the arterial input avoided this problem. There was no donor site morbidity after covering with a split-thickness skin graft. It could be shown that survival of arterialized venous forearm flaps is based on a combination of an arteriovenous shunt and capillary blood flow. In conclusion, we define six rules for a reliable venous forearm flap.

Poster 23: Tracheal Transplantation: Defining the Vascular Territory of the Canine Cranial Thyroid Artery

Poster 23: Tracheal Transplantation: Defining the Vascular Territory of the Canine Cranial Thyroid Artery

A peptidergic component to vagally induced tracheal vasodilation in the dog.

The purpose of the study was to determine the extent that peptidergic afferent and efferent pathways contribute to vagally induced vasodilation in the trachea of the dog. The change in vascular resistance of the tracheal branch of the cranial thyroid artery and the trachealis responses were determined in 28 anesthetized, paralyzed, and mechanically ventilated dogs. After propranolol (2 mg/kg) and phentolamine (1.5 mg/kg), stimulation of the superior laryngeal nerves (NS; 15 Hz, 7 V, 2 ms, 30 s) caused a decrease in vascular resistance of 11.7 +/- 0.8% and a tracheal contraction of 5.2 +/- 4.7 cmH2O. Atropine (1.5 mg/kg) reduced the fall in vascular resistance to 4.7 +/- 0.8% (P less than 0.01), whereas tracheal contraction was abolished. Thiorphan (1.5 mg), a neutral endopeptidase inhibitor, augmented the decrease in vascular resistance (8.8 +/- 0.6%; P less than 0.01) to NS. After hexamethonium (0.5 mg/kg), NS still caused a small decrease in TVR (2.9 +/- 0.9%; P less than 0.05), which was abolished by capsaicin. In atropinized dogs, capsaicin reduced the fall in vascular resistance after NS; the residual vasodilation was virtually abolished by hexamethonium. Acetylcholine (10(-3) mg/kg) decreased vascular resistance (15.7 +/- 3.0%), and the effect was abolished by atropine. We conclude that there is noncholinergic nonadrenergic vagally induced tracheal vasodilation that is peptidergic. The peptidergic vasodilation appears to be mediated by both afferent and efferent pathways.

Measurement of isometric tracheal tension induced by injection into tracheal arterial circulation in vivo

In order to evaluate isometric tension in segments of tracheal smooth muscle in vivo, we first investigated the anatomical distribution of arteries supplying the tracheal smooth muscle in 52 dogs. The patterns of the cranial thyroid arteries were classified into the following four types. A) Common type: the cranial thyroid artery leaves the muscular branch, and then divides into the thyroid branch and the tracheal branch. B) The cranial thyroid artery leaves the tracheal branch, and then divides into the muscular branch and the thyroid branch. C) The cranial thyroid artery divides into the muscular branch, the thyroid branch, and the tracheal branch. D) The cranial thyroid artery divides into the tracheal branch and thyroid branch, and lacks a muscular branch. Out of 52 dogs, 62% had common type bilaterally, and 23% had common type unilaterally. All dogs had a tracheal branch. We demonstrated tracheal smooth muscle contraction by intra-arterial administration of acetylcholine, histamine, 5-hydroxy-tryptamine, phenylephrine, and clonidine using isometric technique in vivo. These results suggest that tracheal contraction induced by sympathetic amines is mediated through two subtypes of receptors, alpha 1 and alpha 2, in tracheal smooth muscle.

Comparative anatomical studies of thyroid and thymic arteries: I. Rat (Rattus norvegicus albinus)

The thyroid and thymic arteries were investigated in 50 male and 50 female rats. In more than 70% of the animals, on both sides the cranial thyroid artery forms a common trunk with the ascending pharyngeal artery. The caudal thyroid artery arises not from the deep cervical but from the pericardiacophrenic artery. It may be replaced, however, by a branch of some other artery, such as the brachiocephalic, subclavian, vertebral, or ascending cervical, suggesting a shift of its origin from the internal thoracic artery to the thyrocervical trunk as in man. All the thoracic lobes of the thymus are supplied directly by a thymic branch of the internal thoracic artery or indirectly by a branch of the pericardiacophrenic artery. More than half of the specimens have a cervical thymic lobe of variable size, which is supplied by a branch of the cranial thyroid, external carotid, and/or occipital arteries. Some of these thymic arteries, except those from the external carotid and occipital arteries, reach the thoracic lobe. The thoracic lobes lacking a cervical lobe may be supplied by the thymic branch arising only from the cranial thyroid artery. Other anomalous arteries supplying the thoracic lobe are derived from the superficial cervical and/or the right common carotid arteries. These results show that the thymic arteries of rats are basically similar to those of man, although they display a clear difference in their frequency and origin.

Involvement of Inhibitory Innervation in Reflex Tracheal Dilatation Induced by Lung Inflation

AbstractWe investigated the involvement of inhibitory innervation in reflex tracheal dilatation (RTD) induced by inflating the lungs in dogs. RTD was inhibited about 50% by 100 μ propranolol injected into the cranial thyroid artery, but was unaffected by adrenalectomy. Residual RTD under β-blockade was abolished by sections of both the bilateral superior laryngeal nerves and spinal cord at the C1 level. These findings suggest that RTD may be mediated by adrenergic innervation and partly by nonadrenergic inhibitory innervation

Involvement of inhibitory innervation in reflex tracheal dilatation induced by lung inflation.

We investigated the involvement of inhibitory innervation in reflex tracheal dilatation (RTD) induced by inflating the lungs in dogs. RTD was inhibited about 50% by 100 micrograms propranolol injected into the cranial thyroid artery, but was unaffected by adrenalectomy. Residual RTD under beta-blockade was abolished by sections of both the bilateral superior laryngeal nerves and spinal cord at the C1 level. These findings suggest that RTD may be mediated by adrenergic innervation and partly by nonadrenergic inhibitory innervation.